The interaction of carbon monoxide with model astrophysical surfaces.

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The interaction of carbon monoxide with model astrophysical surfaces† Mark P. Collings, John W. Dever and Martin R. S. McCoustra* Carbon monoxide (CO) is an important component of the icy mantles that accrete on interstellar dust grains. To develop a better understanding of the physicochemical basis of its infrared spectroscopy, we have studied the interaction of submonolayer coverages of CO with the surface of films of other astrophysically relevant species –

13

CO, carbon dioxide (CO2), ammonia (NH3), methanol (CH3OH) and

water (H2O) – under ultrahigh vacuum and cryogenic (10 K) conditions using reflection-absorption infrared spectroscopy (RAIRS). In support of these measurements, we have performed ab initio calculations of gas phase dimer complexes, and made comparisons to experimental results of gas phase and matrix isolated complexes, which are extensively reported in the literature. The interaction of CO can be cateReceived 23rd September 2013, Accepted 10th December 2013

gorised as occurring via the C atom (CCO bonded), the O atom (OCO bonded) or in a p-bonded configu-

DOI: 10.1039/c3cp54024c

ration. The CCO configuration is characterised by a blue shifted CRO stretch frequency, and is observed for CO adsorbed on 13CO, CO2 and H2O surfaces. From the absence of such a feature from

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this adsorption configuration are not present at the surface of the CH3OH film.

the spectra of CO adsorbed on CH3OH it can be concluded that the dangling OH bonds required for

1. Introduction Carbon monoxide (CO) is the second most abundant molecule in interstellar ices after water (H2O).1,2 This, combined with its spectroscopic sensitivity to its environment and accessibility with terrestrial telescopes, has made it a favoured target species for observations of interstellar ices.3 In support of these observations, CO has been extensively studied in laboratory astrophysics experiments. The mid-infrared spectrum of CO has been measured for the pure solid and a range of astrophysically relevant CO mixtures,4 over a range of temperatures in films with varying thermal histories,5 and in the absence or presence of astrophysically relevant radiation such as UV photons,6,7 protons7 and other cations,8 and electrons.9 CO ice is understood to be present in two distinct interstellar environments:1,3 a ‘non-polar’ environment in which is CO is close to pure, with only minor abundances of weakly interacting species such as nitrogen (N2), oxygen (O2) and carbon dioxide (CO2); and a ‘polar’ environment, where CO is mixed in an ice with hydrogen bonding species such as H2O, methanol (CH3OH) and ammonia (NH3). Such environments may be present as distinct layers in the icy mantles on dust grains, or on different populations of dust grains along a line of sight.

Institute of Chemical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UK. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3cp54024c

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The stretching vibration of the CRO bond is in the midinfrared at around 2140 cm 1. Numerous laboratory studies have shown that for mixtures of CO and H2O co-deposited at temperatures representative of ice mantles on dust grains in dense molecular clouds (10–20 K1,10), the nCO band separates into two features at 2152 cm 1 and 2138 cm 1.11–22 Amorphous solid water (ASW) ice deposited at these temperatures is known to be highly porous.20,23 CO is therefore essentially adsorbed on H2O surfaces in pores throughout the ASW film. The two bands are attributed respectively to CO molecules interacting via the CCO atom with dangling OH bonds at the H2O surface, and to CO molecules interacting with dangling O bonds and with tetrahedrally bound H2O molecules at the H2O surface.24–26 Only by annealing the CO–H2O mixture to temperatures of above B80 K is the single nCO band at 2137 cm 1 for CO in a H2O ‘‘matrix’’ observed. The doublet profile for the nCO band of CO molecules bound to the surface of such compact ASW or to cubic crystalline (Ic) water ice remains.22 In principle, the thin icy mantles on rough dust grain surfaces should have a high enough abundance of molecules at the ice surface to allow their detection, if a spectroscopic band distinct from those of the bulk ice can be identified.27 Despite the high abundance of both CO and H2O in interstellar ices, however, the higher frequency feature of CO on a H2O surface is not observed in astrophysical spectra.3 To further the spectroscopic understanding of this astrophysically important molecule, we have studied the interaction of CO with a range of molecular ice surfaces. For brevity, only the interactions with

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the species most abundant in interstellar ices – H2O, CH3OH, CO2 and NH3 – are discussed in detail in this publication. Data for other species – argon (Ar), krypton (Kr), methane (CH4), ethane (C2H6) and hydrogen sulfide (H2S) are presented in the ESI.† Our infrared spectra are supported by ab initio calculations for 1 : 1 (i.e. gas phase) complexes of CO with each species. Computational studies at higher levels of theory already exist in the literature for most of these complexes.28–53 However, we wish to observe trends in the nature of the intermolecular interactions as the species with which CO is complexed changes. Since previous publications have been performed by differing methods and varying levels of theory, comparison of their results reveals as much about the differences in the computational methods as it does about the differences in the systems of interest. Our relatively low level computations have provided the opportunity to compare a large number of calculations (more than two hundred starting configurations) on ten systems. To perform this number of calculations using state-of-the-art theory may require significant additional effort. We have also considered the wealth of information regarding CO complexes obtained from the extensive literature of gas phase and matrix isolation experiments. The inherent weakness of these intermolecular interactions means that low temperatures are required to stabilise weakly bound complexes for experimental investigation. For gas phase complexes, this can be achieved experimentally using free jet expansion methods,54–68 in which temperatures of a few Kelvin are readily attainable. Complexes at similar temperatures can be isolated in the solid phase within a matrix of a rare gas, or other weakly interacting species such as N2 or O2.28,29,36,69–75 While such matrix isolation spectra are simpler to interpret, they lack the wealth of rotational data that can provide structural information about the complex. Slight species dependent perturbations of the matrix on the complexes also affect such spectra. In some respects, surface adsorption can be considered as intermediate between matrix isolation and the gas phase. It brings new experimental challenges, foremost of which is the low concentration of CO molecules at surface sites. While surface adsorbed molecules are not fully surrounded by the matrix species, a high proportion of the CO population will be perturbed by interaction with multiple surface molecules and by neighbouring CO molecules. As a result, infrared absorption peaks tend to be broader and flatter than those for equivalent CO populations physisorbed on more defined surfaces such as single crystals.

2. Experimental details The experimental apparatus used for reflection–absorption infrared spectroscopy (RAIRS) measurements has been described in detail elsewhere.76 Briefly, it consists of an ultrahigh vacuum (UHV) chamber, with a base pressure of 1 10 10 mbar. The sample was a copper plate coated with a polycrystalline gold film. This could be cooled to 8 K by a closed cycle

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helium cryostat, and radiatively heated to above room temperature while the cryostat was operating. Adsorbate gases were introduced to the chamber at pressures ranging from 5 10 9 to 3 10 7 mbar via glass tubes which were directed at the sample surface, from a distance of roughly 75 mm. Deposition in this manner produces films in which the incoming trajectory of adsorbed molecules is essentially random, avoiding any angle dependent structural effects such as those observed for molecular beam deposition of water.23 CO (99.3%, BOC), 13CO (99%, BOC) and Ar (99.999%, BOC) were used without further purification. CO2 gas was obtained by purifying dry ice (BOC) with repeated freeze–pump–thaw and bulb-to-bulb distillation cycles. CH3OH (analytical grade, Fischer) and H2O (deionised) were further purified by repeated freeze–pump–thaw cycles. RAIRS was performed at an incidence angle of 751 to the surface normal, with an instrument resolution of 1 cm 1 and the co-addition of 1024 scans. Spectra were first recorded for a film of 50 L (where 1 L = 1 10 6 mbar s, uncorrected for ion gauge sensitivity or increased uptake due to the directed dosing method) deposited at 10 K. The rate of uptake by the sample for a given chamber pressure will vary between species, and the exact thickness of layers has not been quantified, however the films are thick enough to ensure that the underlying substrate does not influence the results. CO was then progressively deposited onto these films at 10 K up to a total exposure of 1.0 L and a RAIR spectrum recorded after each deposition. An additional 50 L film of the initial species was then deposited at 10 K before recording the spectrum of the ‘sandwiched’ CO. Deposition of CO at 10 K onto crystalline films of NH3, CH3OH and H2O grown at 80 K, 98 K and 140 K respectively, was also performed. The spectra of the initially deposited amorphous and crystalline films are not shown, but in each case show good agreement with previously published spectra recorded under similar conditions. Further CO deposition experiments were performed (not shown) and the resulting spectra carefully examined to ensure that the 1.0 L CO exposure is below the onset of multilayer growth.

3. Computational details Ab initio computations were performed using the HyperChem (Release 7.0) software package.77 The calculations were based on a Hartree–Fock self-consistent field (HF SCF) method, applying the 6-31G** basis set for all species. CO molecules were positioned around the second molecule in various starting geometries, including those of stable gas phase complexes determined from experiment and of stable geometries predicted in previously published computational studies. Various other geometries, both plausible and implausible, were also tested. Geometry optimisation was performed, using the Polak– Ribiere (conjugate gradient) algorithm, with a root mean square (RMS) gradient of 0.01 kcal Å 1 mol 1 initially set as the termination condition. The starting geometries optimised toward one of, typically, several stable geometries, although in


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some cases the molecules repelled toward infinite separation in a non-bonding interaction. Representative configurations were then further optimised at a RMS gradient of 0.005 kcal Å 1 mol 1, causing some apparently stable geometries to merge into others. No further changes in geometry were observed at more stringent RMS gradients. The energies, CRO bond lengths and vibrational frequencies were calculated for complexes optimised to a RMS gradient of between 0.002 and 0.004 kcal Å 1 mol 1. Imaginary values amongst the predicted frequencies of the normal modes of vibration provided an indication that the complex geometry represented a saddle point on the potential energy surface (PES) rather than a true local minimum. The relative stability of each complex is given by the interaction energy, which was calculated as the difference between the total energy of the complex and the sum of the total energies of the monomers. The basis set superposition error (BSSE) was estimated by a method equivalent to the counterpoise correction proposed by Boys and Bernardi.78 In this method the reference energies of the monomers were calculated with their basis sets derived in the dimer centred basis set. This was achieved by alternately setting the atoms of one molecule as ‘ghost atoms’ in a single point energy calculation. BSSE can cause a significant exaggeration of the stability of the complex, and distort the predicted geometry, although at least in the case of the CO–H2O complex, this geometric distortion has previously been found to be minimal.79

4. Results The nCO regions of the experimental IR spectra are displayed in Fig. 1 and 2 as difference spectra with respect to the initial 50 L deposition, for both the surface adsorbed and sandwiched CO layers. The presence of the additional 50 L layer in the sandwich experiment means that the baseline of the RAIR spectra in the nCO region is usually sloped. Peak positions and peak widths, as measured by the full width half maximum (FWHM), are summarised in Table 1. The interaction of CO with each film is discussed in turn below, however, some trends in behaviour can be identified. As the CO exposure is increased, the peaks tend to become slightly broader, reflecting the increase in disorder due to CO–CO interactions within the overlayer. The centre of the peaks also tend to be shifted slightly toward the vibrational frequency observed for solid CO (2143.0 cm 1), whether this is an increase or decrease, also due to increased CO–CO interaction. The sandwich peaks tend to be broader than those of CO adsorbed on the film surface, again reflecting the greater range of environments in what is effectively a concentrated mixture of CO within the co-adsorbate. The positions of the sandwich peaks tend to occur at lower frequency than those of CO adsorbed on the film surface. No significant differences in the CO spectra were observed for CO adsorbed on crystalline films. To aid in the discussion of the nCO frequency for CO in various environments, peak positions are considered as either


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Fig. 1 RAIR difference spectra of (a, i) 0.5 L of 12CO adsorbed on 50 L of pre-deposited 13CO, and (b, i) 0.5 L of 13CO adsorbed on 50 L of predeposited 12CO; (a, ii) 0.5 L 12CO sandwiched between 50 L depositions of 13 CO, and (b, ii) 0.5 L 13CO sandwiched between 50 L depositions of 12CO; all depositions at 8 K; spectra of the initially deposited 50 L film have been subtracted in (a, i) and (b, i); spectra of the initially deposited 50 L film have been subtracted twice in (a, ii) and (b, ii). Arrows mark the predicted frequencies of nCO vibrations of molecules with CCO-bonded and p-bonded molecules for stable 12CO–12CO complexes in the ab initio calculations (see Section 5). The dotted line marks the position of the nCO vibration for CO adsorbed on an argon surface.

blue shifted (higher frequency) or red shifted (lower frequency) with respect to a reference frequency. For discussion of experimental gas phase studies and computational studies of CO complexes, the nCO frequency of the CO monomer at 2143.271 cm 1 55 is a convenient reference. For various solid phase interactions, where of course a CO monomer is not possible, we adopt the interaction with Ar as a reference. Thus, for discussion of matrix isolation results, the nCO frequency of the (librating) CO monomer at 2138.5 cm 1 69 is the reference; for surface adsorption systems, the reference is the nCO frequency of CO–Ar at 2140.1 cm 1 (Fig. S1(i) and Table S1, ESI†); for sandwich experiments, the reference is the nCO frequency of CO sandwiched in Ar at 2140.4 cm 1 (Fig. S1(ii) and Table S1, ESI†). The ab initio calculations overestimate the nCO frequencies by roughly 15%. In order to make comparisons with experimental measurements of gas phase complexes, the predicted nCO frequencies were scaled by the factor of 0.87776 required to


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Fig. 2 RAIR difference spectra of CO adsorbed on (a) CO2; (b) NH3; (c) CH3OH and (d) H2O; (i) 0.25 L, 0.50 L and 1.0 L of CO adsorbed at 8 K onto a 50 L film of the second species pre-deposited at 8 K, (ii) 1.0 L CO ‘sandwiched’ at 8 K between 50 L deposits of the second species; (iii) 0.5 L CO (in (b) and (c)) or 1.0 L CO (in (d)) adsorbed at 8 K onto a 50 L crystalline films of ammonia, methanol and water deposited at 80 K, 98 K and 140 K respectively; spectra of the initially deposited 50 L film have been subtracted in each case. Arrows mark the predicted frequencies following scaling of raw values by a factor of 0.87666 of nCO vibrations of CCO-bonded, OCO-bonded and p-bonded molecules for stable CO complexes in the ab initio calculations (see Section 5). The dotted line marks the position of the nCO vibration for CO adsorbed on an argon surface.

1.4

frequencies were scaled by the factor of 0.87666 required to correct the predicted frequency of the CO–Ar complex to the experimentally measured value of 2140.1 cm 1. The geometries of the optimised complexes located in the ab initio calculations are shown in Fig. 3–7. The energetic, spectroscopic and bond length data corresponding to these complexes are displayed in Table 2. The literature data for the gas phase and matrix isolated complexes are summarised and compared to the experimental results for CO adsorbed on a film surface obtained here in Table 3.

3.8

4.1

Table 1 Experimental nCO peak positions and widths for 0.5 L of CO adsorbed on, and 1.0 L CO sandwiched in films of various species

Film species 13

CO

CO2 NH3am NH3crys CH3OHam CH3OHcrys H2Oam H2Ocrys

Peak position {FWHM}/cm 1

Shifta/ cm 1

2139.5 2144.6 B2142 2151.7 2141.6 2141.7 2139.3 2139.8 2141.0 2154 2141.1 2153.7

0.6 4.5 B2 11.6 1.5 1.6 0.8 0.3 0.9 B14 1.0 13.6

{4.6} {2.3} {—} {3.5} {7.6} {4.2} {5.9} {7.2} {9.5} {—} {7.6} {8.0}

Sandwich peak position {FWHM}/cm 1

Shiftb/ cm 1

2140.4 {2.2}

0.0

2142.8 {10.5}

2.4

2139.0 — 2136.6 — 2140.3 2154 —

{7.5} — {7.0} — {9.8} {—}

0.1 B14 —

a Relative to nCO for CO adsorbed on Ar at 2140.1 cm 1 (Fig. S1(i) and Table S1, ESI). b Relative to nCO for CO sandwiched in Ar at 2140.4 cm 1 (Fig. S1(ii) and Table S1, ESI).

correct the predicted frequency of the CO monomer to the experimental value of 2143.271 cm 1.55 To allow comparison with the RAIR spectra of surface adsorbed CO, the predicted nCO


12

CO–13CO interaction

The nCO spectra of sub-monolayer depositions of 12CO adsorbed onto a film of 13CO, and of the inverse experiment are shown in Fig. 1a(i) and b(i) respectively. Neither the 12CO or 13CO gases were isotopically pure – the 12CO contains a natural abundance of 1.11% 13CO, and the 13CO contains less than 1% 12CO as an impurity. Since these data are presented as difference spectra, the impurities in the underlying film do not contribute to the spectra, and the observed peaks are due only to the freshly deposited CO and the changes it causes at the surface of the underlying film. The spectrum of submonolayer 12CO adsorbed on 13CO shows a doublet profile with peaks that are blue and


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Paper A summary of results of ab initio calculations for complexes of CO with various species

CO bonding Starting geometries Uncorrected configuration optimisedb stability/kJ mol

Complex


CO monomer

1

—

8 5 2

1.347 1.339

CO CO2 Fig. 4 CCO ORC CO2 ‘T’-shaped; (i) OCO CRO CO2 ‘T’-shaped; (ii) CRO O–CO ‘L’-shaped; (iii)a p

10 4 5 1

CO NH3 Fig. 5 ORCJH–NH2 compact; (i) ORC H–NH2 linear; (ii) CROJNH3 flat; (iii)a

p CCO p

CO CH3OH Fig. 6 ORC H–OCH3 linear; (i) CROJHO–CH3 flat; (ii) ORCJH–OCH3 compact; (iii) ORCJHO–CH3 flat; (iv)a CRO H–OCH3 linear; (v) CRO CH3OH linear; (vi) ORC H–CH2OH linear; (vii) ORC CH3OH linear; (viii) CO H2O Fig. 7 ORC H–OH linear; (i) ORCJH–OH compact; (ii) CRO H–OH linear; (iii) CROJOH2 flat; (iv)a

CO CO Fig. 3 Offset anti-parallel; 3a(i) ‘L’-shaped; (ii) base (ii) stem

1

Corrected stability/kJ mol

Corrected Corrected CRO bond (surface) (gas phase) length/Å nCO/cm 1 c nCO/cm 1d 1.11365

—

2143.27

0.082 0.028

1.11398 1.11380 1.11339

2138.3 2139.1 2142.8

2140.94 2141.79 2145.47

3.388 2.707 1.433

1.374 1.537 0.539

1.11284 1.11418 1.11373

2147.0 2135.8 2139.7

2149.62 2138.49 2142.32

14 9 3 2

5.273 3.297 1.132

2.489 1.120 1.215

1.11467 1.11280 1.11377

2132.6 2147.4 2139.2

2135.27 2150.05 2141.80

CCO p p p OCO OCO CCO CCO

23 2 6 4 2 4 3 1 1

6.601 5.402 5.337 5.015 4.947 1.176 1.141 1.060

3.704 2.537 2.273 2.458 3.611 0.433 0.001 0.059

1.11193 1.11469 1.11493 1.11429 1.11472 1.11401 1.11328 1.11329

2155.2 2132.2 2130.0 2135.9 2131.0 2137.8 2143.5 2143.5

2157.83 2134.85 2132.67 2138.55 2133.60 2140.43 2146.18 2146.20

CCO p OCO p

17 4 6 6 1

6.435 5.333 4.976 4.802

3.613 2.161 3.684 1.087

1.11194 1.11495 1.11477 1.11482

2154.8 2130.2 2130.8 2131.5

2157.47 2132.80 2133.48 2134.14

p p CCO

—

1

a Saddle point geometries. b Where the sum of the optimisations for individual complexes does not equal the total number of optimisations for that system then some starting geometries optimised to a non-bonding configuration. c Raw frequencies are scaled by a factor of 0.87666; see Section 4. d Raw frequencies are scaled by a factor of 0.87776; see Section 4.

Table 3 A summary of the shifts in nCO due to interaction of CO with other species for gas phase complexes, matrix isolated complexes and surface adsorption

Interaction CO–CO CO–CO2 CO–NH3 CO–CH3OH CO–H2O

Gas phasea 55

1.04, 0.04 5.4158 0.662 11.664 10.767

Matrix isolationb 69

1.6 4.8, 4.5, 7.4, 5.4, — 10.974

3.170 1.972

Surface adsorptionc 4.5, 0.6 11.6, B2 1.5 0.8 B14, 0.9

a

Relative to nCO for the gas phase CO–Ar complex at 2142.83 cm 1.54 Value for the CO complex in an argon matrix relative to nCO for a librating CO monomer in an argon matrix at 2138.5 cm 1.69 c Adsorption on an amorphous film, relative to nCO of CO on an argon film at 2140.1 cm 1; this work. b

red shifted from the CO–Ar reference by 4.5 and 0.6 cm 1, respectively. The 13CO spectrum shows a similar profile (with peaks at 2097.7 and 2091.6 cm 1). This demonstrates that CO molecules adopt two spectroscopically distinct configurations at the surface of a film. Furthermore, the underlying film in both cases shows a doublet of loss features at equivalent frequencies with a single central peak, corresponding to molecules that were previously at the surface now present in bulk sites.


In order to view the sandwich spectra as difference spectra it is necessary to subtract the spectrum of the initially deposited film twice, since neither the 12CO and 13CO gases are isotopically pure. As is evident from the spectra in Fig. 1a(ii) and b(ii), this method is successful for small amounts of 12CO and 13CO present dilutely within 13CO and 12CO films. However, due to optical effects, the nCO band of a 100 L deposition of 12CO is definitely not equivalent to twice the nCO band of a 50 L deposition. Therefore the nCO regions of the bulk CO material in these difference spectra do not provide any useful information, and they have been omitted. The spectra of 12CO sandwiched between 13CO layers shows no shift compared to the CO in Ar sandwich spectrum. The (12C16O)2 and (13C16O)2 complexes have been experimentally studied in the gas phase by infrared, microwave and radiofrequency spectroscopy.55–57 Evidence was obtained for two almost isoenergetic isomers of the 12C16O dimer,55 for which the origin of the nCO frequencies are shifted by 0.62 and 0.40 cm 1 with respect to the CO monomer. However, interpreting the structure of the dimer has proved difficult due to the non-rigidity of the complex, in which many minima exist on the PES separated by small barriers. ‘T’-shaped geometries with binding via the CCO atom for the true minimum and via


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Fig. 3 Local minimum geometries for CO–CO complexes. Marked bond distances are in angstroms.

the OCO atom for the isomer with 0.010 kJ mol 1 higher and lower vibrational frequency energy were initially suggested.55 Similarly, computations are complicated by the difficulty in accurately calculating the dipole moment for CO,30 and studies using a variety of methods25,29–33 have generated a range of predictions for the CO dimer, none of which have reproduced the experimental gas phase results clearly. The stability of the global minimum varied between 0.70 and 1.85 kJ mol 1, and was found to have a C C offset antiparallel geometry in the two most recent studies. Our ab initio calculations found two stable geometries for the (CO)2 complex, both of which were planar. The O O offset anti-parallel complex (Fig. 3(i)) has been found in most of the previous studies. The ‘L’ shaped complex (Fig. 3(ii)) has not been identified previously, but is not dissimilar to the C-interacting ‘T’-shaped complex. These had an equivalent uncorrected stability, although the former was obtained from more than twice as many starting geometries. However, following BSSE correction, the predicted stabilities are very low. The nCO frequency for the antiparallel complex was red shifted by 2.3 cm 1 from that of the CO monomer. In the ‘L’ shaped complex the CO molecules are not equivalent, and hence have different nCO frequencies; the ‘base’ is shows a red shift of 1.48 cm 1 and the ‘stem’ a blue shift 2.20 cm 1. The direction of these shifts is consistent with predictions in previous computational studies,25,29 although somewhat larger. Matrix isolation experiments with CO diluted in Ar have been published by numerous authors over the past five decades.28,29,69 However, the assignment of peaks, the presence or absence of some peaks, and the possible effects of impurities such as H2O are controversial. We consider the most reliable of these papers to be that of Abe et al.,69 who assign a single peak to the CO dimer, blue shifted by 1.6 cm 1 with respect to the CO–Ar complex. 4.2

CO–CO2 interaction

The nCO feature for CO adsorbed onto a CO2 film is broad, strongly blue shifted and highly asymmetric (Fig. 2a(i)), tailing off to the low frequency side, suggesting that more than one adsorption configuration contributes to the profile. The main component of the feature is blue shifted by 11.6 cm 1 at low coverage (0.25 L) with respect to CO–Ar, although the extent of the shift appears reduced at higher CO exposure. The weaker component can not be deconvoluted, but is perhaps present with a blue shift of around 2 cm 1. The nCO peak for the sandwiched CO is broad, with a FWHM of 10.5 cm 1, and is blue shifted by 2.4 cm 1 with respect to the CO in Ar sandwich


Fig. 4 Local minimum geometries for CO–CO2 complexes. Marked bond distances are in angstroms. Complexes found to be saddle point geometries based on vibrational analysis are marked *.

(Fig. 2a(ii)), in good agreement with for the published spectra for equimolar mixtures.80 The origin of the nCO band of the gas phase CO–CO2 complex measured in pulsed free jet molecular beam experiments is blue shifted by 4.97 cm 1 from that of the CO monomer.58 The structure of the complex has been shown to be ‘T’-shaped, with interaction between the two carbon atoms.58–61 This C CCO ‘T’-shaped structure was found to be the global minimum geometry for the CO–CO2 complex in computational studies using molecular mechanics (MM)34 and MP2 ab initio35 methods. Stabilities of 4.75 and 3.18 kJ mol 1, respectively and nCO frequencies blue shifted by 1.4 and 3.6 cm 1 respectively were predicted. Both studies also found a less stable C OCO ‘T’-shaped complex, for which the nCO frequency was almost unshifted in the MM study and red shifted by 0.4 cm 1 in the MP2 study. Two further stable structures with the CRO molecule lying alongside a OCQO bond in either a parallel or antiparallel configuration were also found in the MM study, with predicted nCO shifts of 0.18 cm 1 and 1.42 cm 1 respectively. Three nCO bands with shifts of 4.8, 4.3 and 3.1 cm 1 were observed for the CO–CO2 complex isolated in an argon matrix,70,71 which were linked with distinct complex geometries. An extension of the MM study of gas phase CO–CO2 complexes that took account of the argon matrix environment34 identified similar configurations for complexes but found the direction of the nCO band shifts is reversed, a result that is at odds with essentially all of the other computations in the field. The computations performed here identified three stable geometries for the gas phase CO–CO2 complex, all of which were planar. The C C ‘T’ shaped structure (Fig. 4(i)) was the most stable uncorrected complex, although following BSSE correction the C OCO ‘T’ shaped complex (Fig. 4(ii)) became the global minimum, with a stability of 1.54 kJ mol 1. Vibrational analysis of the ‘L’-shaped complex (Fig. 4(iii)) indicates that it is a saddle point geometry. The predicted shifts in nCO frequency with respect to the CO monomer for these complexes were 6.35, 4.78 and 0.95 cm 1 respectively. 4.3

CO–NH3 interaction

The nCO spectra of CO adsorbed on an amorphous NH3 film (Fig. 2b(i)) show a single peak which is blue shifted by 1.5 cm 1,


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Fig. 5 Local minimum geometries for CO–NH3 complexes. Marked bond distances are in angstroms. Complexes found to be saddle point geometries based on vibrational analysis are marked *.

and show minimal variation in position and width as the coverage increases. On a crystalline NH3 film, the peak shows a similar blue shift, but is somewhat less broad (Fig. 2b(iii)). In contrast, the nCO peak of CO sandwiched in amorphous NH3 shows a small red shift of 1.4 cm 1 with respect to CO sandwiched in Ar. The gas phase CO–NH3 complex in free jet expansions has been studied spectroscopically in the microwave,61 and millimetre wave and infrared62 regions. The CO molecule was found to lie above the N atom, with its C atom tilted slightly toward the NH3 molecule.61 The origin of the nCO band was found to be blue shifted by roughly 0.2 cm 1 from the frequency of the monomer.62 The nCO feature observed for the CO–NH3 complex isolated in an argon matrix after photolytic decomposition of formamide showed a main peak red shifted by 1.9 cm 1 from the position of the band for the CO monomer in argon, with two weaker features blue shifted by 5.4 and 7.4 cm 1.36,72 The accompanying MP2 level calculations linked the red shifted peak to a global minimum linear H2N–H OC complex with a stability of 3.5 kJ mol 1, and the two weaker peaks to a linear OC H–NH2 complex. Other computational studies found similar complex configurations, in addition to compact geometries with the CRO bond lying parallel to an N–H bond and the H–N–H plane.37,38 Three stable geometries of the CO–NH3 complex were identified in our ab initio computations. With a BSSE corrected stability of 2.49 kJ mol 1, the most stable is a compact geometry with the CRO bond almost parallel to a N–H bond (Fig. 5(i)), similar in structure to a complex found in the earliest computations.37 A linear OC H–NH2 complex was also found (Fig. 5(ii)), although the stability was less than half that of the global minimum. The nCO frequencies of these complexes shifted by 8.00 cm 1 and 6.78 cm 1, respectively. A third weakly bound compact complex with CO lying under the NH3 molecule was also found (Fig. 5(iii)). However, after BSSE correction this structure was found to be unstable, and vibrational analysis indicated that it is a saddle point geometry. 4.4

CO–CH3OH interaction

The nCO peak for CO adsorbed on amorphous methanol (Fig. 2c(i)) is broad, showing a small red shift of 0.8 cm 1 compared to the nCO frequency of CO on Ar, and little variation in position and width as the coverage increases. On the crystalline methanol film (Fig. 2c(iii)), the red shift of the nCO


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frequency was slightly less, and the peak was slightly broader. For CO sandwiched between layers of amorphous methanol (Fig. 2c(ii)), the nCO band was red shifted by 3.8 cm 1, but in contrast to other systems, is not appreciably broadened. This spectrum shows reasonable agreement with previously published data for equimolar mixtures of CO and CH3OH.81 The gas phase CO–CH3OH complex has been studied in molecular beam experiments by both microwave63 and infrared64 spectroscopy. The structure determined was a complex with hydrogen bond between the CCO and Hhydroxyl atoms; the CO molecule lay in the plane of the COH group, and was tilted slightly toward a Hmethyl atom in a cis position.63 The origin of the nCO band was found to be blue shifted by 11.2 cm 1 with respect to the CO monomer.64 Although matrix isolation experiments have been performed on the CO–CH3OH complex in argon73 and on methanol in a CO matrix,82 the influence of complex formation on the CO molecule was not discussed in these publications. The linear ORC H–OCH3 complex was also found to be the global minimum in computational studies using self consistent field theory and MP2 theory at varying levels of calculation, although with the in-plane Hmethyl atom in the trans position.39 A stability of 12.00 kJ mol 1 and a nCO frequency blue shifted by 17.5 cm 1 with respect to the CO monomer were predicted. Other stable geometries reported were a linear CRO H–OCH3 complex, and configurations with the CO molecule lying flat between the OCH3OH lone electron pair with either CCO or OCO atom pointing toward the methyl group, with nCO shifts of 10.6, 9.1 and 4.8 cm 1, respectively. More recently, the nCO frequencies of the hydrogen bonded complexes were tested using MP2 and DFT methods, predicting blue shifts in the 11–21 cm 1 range for the CCO-bonded complex and 1 to 8 cm 1 range for the OCO-bonded complex.50 A total of eight stable geometries were identified in our ab initio computations. In agreement with the previously published computations, the linear ORC H–OCH3 geometry was the most stable (Fig. 6(i)), with a stability of 3.07 kJ mol 1 following BSSE correction and a nCO frequency blue shifted by 14.56 cm 1. However, this complex was relatively infrequently reached during geometry optimisation. The previously reported linear CRO H–OCH3 geometry (Fig. 6(v)) and p-bonded CROJHO–CH3 geometry, with the OCO directed toward the methyl group (Fig. 6(ii)) were also identified here. While the p-bonded ORCJHO–CH3 geometry with the OCO atom directed away from the methyl group (Fig. 6(iv)) was also predicted, vibrational analysis identified it as a saddle point geometry. A fifth strongly bound complex was predicted, in which the CRO molecule lies parallel to the O–H but is nearly 301 out of the COH plane (Fig. 6(iii)). The predicted nCO frequencies for these four complexes are all strongly red shifted from that of the CO monomer. Three complexes in which the CO molecule interacts weakly with the methyl group were also identified, although only the geometry in which the CO molecule interacts linearly with the Cmethyl atom via its OCO atom (Fig. 6(vi)) retains stability after BSSE correction. In all eight geometries, the in-plane Hmethyl atom adopts the trans position. A geometry similar to that suggested from gas phase experiments63


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Fig. 6 Local minimum geometries for CO–CH3OH complexes. Marked bond distances are in angstroms. Complexes found to be saddle point geometries based on vibrational analysis are marked *.

was tested, but methyl group rapidly rotated from the cis position to trans position during geometry optimisation. 4.5

CO–H2O interaction

For CO adsorbed on the amorphous water surface, two broad peaks are observed in the nCO band (Fig. 2d(i)). The positions of these peaks are blue shifted by 14 and 0.9 cm 1, in agreement with our previously published results, although we demonstrated previously that the higher frequency peak shifts to lower frequency as the surface concentration of CO molecules rises.22 Since the two peaks are not fully resolved, the FWHM of the higher frequency peak can not be measured. Little change is evident in the sandwich spectrum (Fig. 2d(ii)). The lower frequency peak shifts slightly to the red and broadens a little. It shows good agreement with previously published nCO spectra of co-deposited CO and H2O.11–22,42 Both peaks are also present in the nCO spectrum of CO on the crystalline H2O surface (Fig. 2d(iii)). The peak positions unchanged within experimental error and slightly sharper, again in agreement with our previous results.22 Experimental studies of the gas phase CO–H2O complex have been performed in the microwave,65,66 far-infrared66 and infrared67,68 regions. The complex was found to have a planar CCO–hydrogen bonded structure in which the CRO bond is tilted away from linearity with H–O bond by roughly 111.65 The origin of the nCO band for CO in this complex was found to be blue shifted by 10.33 cm 1.67 The CO–H2O complex has also been observed experimentally in numerous matrix isolation


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experiments by co-deposition of H2O, CO and the matrix material.74,75 A single peak attributable to the complex, with a nCO frequency strongly blue shifted from that of the CO monomer in the respective matrix was observed in all cases. In an argon matrix, this shift is 10.9 cm 1.74,75 A single blue shifted peak was also observed for the CO–H2O complex formed by photolytic decomposition of formic acid in an argon matrix,83,84 and photolytic reaction of formaldehyde with O2 matrix molecules.85–87 However, and additional red shifted nCO peak, due to an additional complex which is unstable at higher temperatures, is observed when formic acid photolytically decomposed in krypton or xenon matrices83,84 The CO–H2O complex has been studied computationally by a range of computational techniques in numerous studies over the years.36,40–53 Almost all found the global minimum geometry to be a planar linear CCO–hydrogen bonded complex with similar structure to that determined from gas phase experiments. The predicted stability of the complex is 6.5 kJ mol 1 when averaged across the studies. Most of these studies also identified a stable linear OCO–hydrogen bonded complex, with a stability of typically about half of the global minimum. The nCO frequency of the CCO–hydrogen bonded is strongly blue shifted in all predictions, typically by 10–13 cm 1, and the OCO– hydrogen bonded complex typically red shifted by less than 2 cm 1. A recent publication has extended DFT calculations to study the CCO– and OCO–hydrogen bonded complexes when solvated in argon, krypton and xenon matrices.52 The complexes showed very little perturbation upon solvation, and the predicted shift of the nCO for the CO monomer and the two complexes to increasingly lower frequency as the dielectric constant of the solvent increased matched the experimentally observed trend. These calculations were used to support the assignment of the blue shifted and red shifted peaks in the nCO spectrum to the CCO– and OCO–hydrogen bonded complexes, respectively. Our ab initio computations found four stable geometries for the CO–H2O complex. After correction for BSSE, the two linear hydrogen bonded complexes had very similar predicted stabilities of roughly 3.6 kJ mol 1, with the OCO-bonded complex slightly favoured. The predicted nCO frequencies for the CCOand OCO-bonded complexes (Fig. 7(i) and (iii)) were 14.2 cm 1 and 9.79 cm 1, respectively. A compact complex with the O–H and CRO bonds close to parallel (Fig. 7(ii)) was also identified,

Fig. 7 Local minimum geometries for CO–H2O complexes. Marked bond distances are in angstroms. Complexes found to be saddle point geometries based on vibrational analysis are marked *.


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although in contrast to the similar compact complexes previously reported,37,40,49 we found that the plane of the tetracycle was roughly 451 to the plane of the H–O–H bond. The predicted nCO frequency for this complex was also strongly red shifted. The least stable geometry, in which the CO and H2O molecules were in parallel planes with the centre of mass of the CO molecule above the OH2O atom (Fig. 7(iv)), was found to be a saddle point geometry. Ternary complexes of H2O and CO have also been studied, both experimentally and theoretically. In particular, the CO–(H2O)2 complex is interesting because it represents the first step between the CO–H2O complex and adsorption of CO on a H2O surface. Computations predict that the blue and red shifts of the nCO frequency of CCO–hydrogen bonded and OCO–hydrogen CO molecules respectively are enhanced by their interaction with a second water molecule in a cyclic complex.53 This result is in qualitative agreement with calculations for CO molecules interacting with multiple H2O molecules on an amorphous H2O surface.26 In experimental studies where glyoxal (H2C2O2) was photolytically reacted with molecules of the O2 matrix,86,87 features with a nCO frequency blue shifted by 24–29 cm 1 relative to the CO monomer were assigned to a CO–(H2O)2 complex of unspecified geometry.

5. Discussion 5.1

Configuration of CO interactions

The orientation of the CO molecule in its stable interactions with other species can be seen to cause systematic variations in the stability, bond length and nCO frequency. The complexes can be conveniently categorised as interacting via the C atom of the CO molecule (CCO-bonded), via the O atom of the CO molecule (OCO-bonded), or in a p-bonded configuration. Fig. 8 plots the stability and CRO bond length of the nCO vibration against the predicted frequency of the nCO vibration for the 39 stable CO complexes identified in our ab initio calculations, including data both for the systems discussed above and those presented in the ESI,† with data points grouped for CCO-bonded, OCO-bonded and p-bonded complexes. For all of the CCO-bonded complexes the frequency of the nCO vibration is predicted to be blue shifted relative to that for the CO monomer. In contrast, all of the OCO-bonded and p-bonded complexes show a red shift in the predicted nCO frequency. The stability of the complex correlates well with increasing shift in the nCO frequency, regardless of whether this shift is to the red or the blue (Fig. 8a). A near-linear relationship between the nCO frequency and the CRO bond length is evident (Fig. 8b), with the bond length decreasing as the nCO frequency increases. 5.2

Comparison of ab initio calculations with literature data

On the whole, the predictions of the ab initio calculations reproduce the experimental results for gas phase complexes quite well, despite the relatively low level of the theory applied. For each system the ab initio calculations reproduce


Fig. 8 (a) Stability, and (b) CRO bond length for complexes at local minimum geometries in ab initio calculations as a function of predicted nCO frequency; data from Table 2 and Table S2 in the ESI;† circles: CCO-bonded complexes; diamonds: OCO-bonded complexes; squares: p-bonded complexes; solid symbols: BSSE corrected stabilities, hollow symbols: uncorrected stabilities; lines of best fit to the BSSE corrected (solid) and uncorrected (dashed) data are provided to guide the eye. The large hollow star symbols mark the values for the CO monomer.

the experimentally determined geometry of the stable gas phase complex (or something similar), and correctly predict the direction and approximate size of the shift in the nCO frequency. However, for the CO2–CO and H2O–CO systems, the experimentally determined geometry is not the global minimum in the calculations after BSSE correction. As discussed above, there is considerable variation in the stabilities predicted for various complexes in the numerous previously published computational studies using assorted methods. Our ab initio calculations consistently predicted stabilities somewhat lower than those typically reported in the literature, falling for example at the lower end of the wide range of published values for the CO–H2O complex. This appears to be a systematic behaviour of the ab initio method. The trends in predicted nCO frequency described above also find support from previous computations. All CCO-bonded complexes for which the nCO frequency was previously calculated showed a blue shifted nCO vibration. A red shifted nCO vibration was predicted for all OCO-bonded complexes for which it was previously calculated, although not generally as large as predicted in our calculations. Relatively few p-bonded


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complexes were analysed for their vibrational frequencies, and these predicted minimal shifts in the nCO frequency.


5.3

Interpretation of IR spectra of solid phase CO interactions

The qualitative success of the ab initio calculations in reproducing the experimental results for gas phase complexes provides a basis for their use in interpreting the IR spectra of CO complexes in the condensed phase, both for CO adsorbed on film surfaces and co-absorbed with other species within matrices of rare gases. Table 3 compares the shift of the nCO frequency for the various CO complexes in gas phase, matrix isolation and surface adsorption experiments. For the gas phase complexes, only the CO dimer showed evidence for two geometries, which were found to be almost isoenergetic.55 Clearly, in such experiments, the complexes can overcome the energy barriers between local minima to find the global minimum geometry. This is not always the case for the matrix isolated complexes, where nCO frequencies have been assigned to more than one geometry in the CO–CO2,70,71 CO–NH336,72 and CO–H2O83,84 systems. Similarly, the CO–13CO, CO–CO2 and CO–H2O surface adsorption systems all show evidence of two nCO peaks, which must be due to (at least) two distinct types of adsorption geometry. It is noteworthy that in some cases of the ab initio calculations, the global minimum geometry was not that most frequently obtained during the geometry optimisation process. While this method is by no means a comprehensive test of the PES, it does demonstrate that the geometry of the well, in addition to its depth, may influence the populations of the corresponding geometries, such that the most populated geometry is not necessarily the most stable. The blue shifted nCO vibrations evident in the 12CO–13CO, CO–CO2 and CO–H2O surface adsorption systems can be attributed to CCO-bonded molecules at the film surface. In each case, the size of the shift is greater for the surface adsorption system than has been measured for CCO-bonded matrix isolated and gas phase complexes. For the CO–H2O interaction, this larger blue shift upon surface adsorption is due to CO molecules interacting with more than one H2O molecule.22,26 The effect is also evident for CO–(H2O)2 clusters for which a blue shift in the nCO frequency of 20 cm 1 is predicted by computations,53 and of up to 29 cm 1 is reported in matrix isolation experiments.85,86 The CO–H2O, CO–NH3 and CO–CO2 systems show a red shifted nCO vibration in matrix isolation experiments that has been linked via computational studies with a OCO-bonded complex, although for the CO–H2O system the complex is not stable during annealing. However, no complexes with a strongly red shifted nCO vibration are observed in the gas phase. Those that are not CCO-bonded show nCO vibrations with little shift from that of the CO monomer, and are found to have p-bonded CO molecules in each case. Similarly, there is no evidence of any surface adsorbed complexes with a significant red shift in the nCO vibration. This suggests that CO molecules do not form OCO-bonded interactions with any of the surfaces investigated. All of the surface adsorption systems show a component of the nCO vibration which shows little shift. For the CO–H2O and


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CO–13CO systems, there is a resolved doublet with a blue shifted and an unshifted peak. The blue shifted peak in the CO–CO2 system clearly shows a weak unresolved component with an unshifted frequency. The CO–NH3 and CO–CH3OH systems show only a single unshifted peak, although given the width of these absorption bands, the presence of multiple overlapping components due to distinct adsorption configurations can not be ruled out. By comparison with the experimentally determined structures of gas phase complexes for which the shift in nCO frequency is minimal, the unshifted component of nCO vibration in the surface adsorption experiments is consistent with p-bonded CO molecules. While our ab initio calculations predict that such complexes should show a nCO frequency that is red shifted in proportion to the adsorption strength, there is no experimental evidence and little computational support from previous studies to corroborate this. It appears that the level of theory in our calculations is insufficient to accurately determine vibrational shifts for a CO molecule interacting primarily through its p* orbitals. The widths of the nCO peaks in the surface adsorption experiments are large compared to the size of the predicted shifts for the weaker interactions. Therefore, the possibility that weakly interacting CCO-bonded or OCO-bonded complexes contribute to the unshifted surface adsorption nCO vibrations, in part or in full, can not be ruled out. In the CO–H2O system, however, no such weakly interacting configurations were predicted, and the model of CO adsorption in which the blue shifted peak results from CCO-bonded molecules and the unshifted peak from p-bonded CO molecules is consistent with previous computational studies of CO interactions with water films.19,24–26 Extension of this model of CO–H2O interactions to include CO interacting with other species therefore seems reasonable. Further support for the interpretation of CO bonding that we present can be found in the extensive literature for infrared spectroscopy of CO adsorption on refractory surfaces. It is well known that CCO-bound and OCO-bound species coexist while interacting with the cation sites on the surfaces of a wide range of metal oxide and zeolite surfaces.88,89 The CCO-bound species show a blue shift, which is generally much stronger than those observed here due to a combination of the electrostatic Stark effect and the wall effect (repulsion due to the vibration of the CO molecule against a rigid surface),90 while the OCO-bound species shows a strong red shift. The adsorption of CO on silanol groups at the surface of silica and related materials shows even greater similarity.91–95 The precise position of peaks is dependent on the surface, but the nCO features in the 2154 cm 1, 2131 cm 1 and 2138 cm 1 regions are attributed, with computational support50,93 to SiOH CRO, SiOH ORC and physisorbed CO complexes respectively. These experiments were performed with substrates held in infrared cells at much higher temperature (>80 K) and adsorption maintained by equilibrium pressures of CO (typically several mbar). These differences in experimental conditions may account for the observation of the OCO-bound species in such experiments but its absence in our UHV spectra.


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The deposition of the overlayer in the ‘sandwich’ experiments results in small red shift in the observed nCO frequency for each case, except for the CO–H2O system where the CO molecules remain essentially surface adsorbed due to the porosity of the H2O film.23 Similarly, mixtures of CO and H2O co-deposited at 10 K retain the doublet structure in the spectrum of the nCO vibration characteristic of CO on the surface of H2O.20,22 Upon annealing such mixtures to above 80 K, CO trapped within water matrix displays a nCO frequency that is red shifted by 1.5 cm 1 from the lower frequency peak in the spectrum of CO on the H2O surface.11,12,20 This shift is a general effect, also observed in the ‘sandwich’ experiments shown in the ESI,† and evident in matrix isolation experiments. Being surrounded by other molecules must inevitably result in electron density leaking into the CO p* orbitals, causing a slight destabilisation of the bond and a reduction in the nCO frequency. 5.4

Characteristics of the underlying films

In this publication, we have not attempted to analyse the effects of CO adsorption on the spectral features of the underlying film. Such effects can be readily observed in difference spectra. Such analyses have proved invaluable in studying the adsorption of CO and other species on H2O surfaces, particularly through study of shifts in the dangling OH bond peaks of H2O molecules at the film surface.41,42 However, the spectra of water films are far better understood than those of films of the other species we have studied here, through the extensive work of a great number of research groups over many years. The strength of the interaction between CO and each of the film species is less than that between the molecules in the film. Therefore, it is not energetically favourable for molecules in the surface of the film to rearrange to accommodate even the strongest CO surface interaction. The CO molecules therefore sample a range of sites on the surface, including those suitable for upright adsorption configurations, such as the dangling OH bonds on water surfaces, and those where the CO molecule will adopt p-bonded configuration. The absence of any OCO-bonded interactions demonstrates that for adsorption onto surfaces at 10 K at sites suitable for an upright interaction, the room temperature CO molecules have sufficient energy to allow relaxation into the more stable CCO-configuration on the timescale of the adsorption process. However, the simultaneous presence of both CCO-bonded and p-bonded interactions indicates either that the adsorbing CO molecules have insufficient energy to diffuse across the surface to find the most stable sites, or that the two configurations have comparable adsorption energies. For the CO–H2O surface adsorption, both are true.22 We have not attempted to measure relative adsorption strengths for CO in the two types adsorption configurations for the other systems, but it is likely that CO is unable to diffuse across any surface at 10 K. The CO–CH3OH system is one for which there is an obvious discrepancy between the RAIR spectra and previous gas phase experimental results. The results of computational studies, in both this and previous publications, and gas-phase cluster


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experiments suggest that CCO-bonded linear ORC H–OCH3 complex is the most stable. However, the RAIR spectra of CO adsorbed on both amorphous and crystalline methanol show a single nCO peak which is slightly red shifted and inconsistent with a CCO-bonded interaction. It is possible that the hydrogen bonding network in methanol ice causes sufficient distortion of the methanol molecules to make the linear ORC H–OCH3 geometry unstable. However, this seems unlikely, since in the hydrogen bonding network in water ice is stronger and no similar effect is apparent. Orientation of the methanol molecules such that dangling OH bonds are never presented at the surface is a more likely alternative explanation. Previous studies of amorphous methanol nanoparticles have drawn the same conclusion.96,97 Similarly, no CCO-bonded interactions are evident on the NH3 surface. While the experimentally determined gas phase complex has ORCJH–NH2 geometry, computational and matrix isolation studies demonstrate that the linear ORC H–NH2 complex is stable. With the model of CO adsorption described above, we should expect to see a blue shifted nCO peak if dangling N–H bonds are present at the surface. The absence of a CCO-bonded interaction on these surfaces demonstrates that, like CO, the CH3OH and NH3 molecules have sufficient energy to undergo some reorientation at the surface during deposition at 10 K, but not to form a crystalline film. Dangling OH bonds at the surface of H2O films are readily identifiable in infrared spectra,41,42 but the equivalent dangling bonds have not been reported for CH3OH98,99 and NH3100,101 films.

6. Astrophysical implications CO is the second most abundant species in interstellar ices. Since H2O is the most abundant species in interstellar ices, it might be expected that the interactions of CO with H2O would make a significant contribution to the spectroscopy of CO ice. Similarly, since CO2 and CH3OH are formed in the ice mantles by reactions of CO, the interactions of CO with these species would also be expected to influence the nCO band structure. CO is sufficiently abundant in the solid phase that detection of CO adsorbed at an ice surface is feasible, if a suitably characteristic vibrational frequency exists. The blue shifted peaks of CCO bonded molecules on H2O and CO2 surfaces show the greatest separation from bulk CO vibrations, and therefore offer the best prospect for astronomical detection. In observations of 39 young stellar objects by Pontoppidan et al.,3 the nCO feature stretches from roughly 2125 to 2147 cm 1. In analysis, it was demonstrated that the spectrum toward each object can be decomposed into three bands with defined frequencies and widths, but varying intensities. The strongest component in most sources is at 2139.9 cm 1, and is assigned to pure or near-pure CO ice. A higher frequency component at 2143.7 cm 1 is assigned to the longitudinal optical (LO) component of solid CO, indicating some degree of linear polarisation of the background infrared source. The lower frequency component at 2136.5 cm 1, the broadest of the three, is assigned to CO


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within the matrix of polar ice. The contribution of a more strongly blue shifted component, which would indicate a population of CCO-bonded molecules, is not required. Amorphous water ice formed at temperatures representative of interstellar ice mantles (B10 K) has a highly porous structure.20,23 CO molecules in a co-adsorbed mixture with H2O at these temperatures effectively resides on pore surfaces. The surprising absence of a feature at 2152 cm 1 due to CCO-bonded molecules on pore surfaces has long been noted in the astrophysical literature.11,12 The loss of porosity due to thermal processing,11–22 processing by cosmic rays102,103 or processing by UV photons,103 could account for the absence of the feature, although corroborative evidence for annealing or extensive radiative processing of interstellar ices is lacking. Localised annealing of water ice as heat is released during the reactive formation of H2104 has also been put forward. Fraser et al.105 suggested that competition for dangling bond sites of porous H2O from other more strongly bound species may also explain the absence of the 2152 cm 1 feature. Most recently, Cuppen et al.81 have demonstrated that the spectrum of the polar component of the nCO band is consistent with CO–CH3OH mixtures. Thus, in interstellar ices CO is separated into populations in near-pure and methanol rich environments. Within each population, some CO molecules must reside at a surface. However, the results here demonstrate that for neither population can the nCO frequencies of surface and bulk CO molecules be distinguished. Although gas phase and computational studies identify a hydrogen bonded ORC H–OCH3 complex with a strongly blue shifted nCO frequency, this complex does not exist for CO on a CH3OH surface. Without such a feature, it is not possible to use these results to further constrain the CO–CH3OH mixtures that may be responsible for the polar component of the nCO band in astrophysical spectra.

7. Summary and conclusions After studying the interaction of CO with a range of molecular ice surfaces using RAIRS, with the support of ab initio calculations of gas phase complexes and consideration of experimental studies of CO complexes in gas phase and matrix isolation experiments from literature, a general trend that relates the frequency of the CRO stretch vibration with the configuration of the interaction of the CO molecule emerges. The interaction can be characterised as occurring via either the C atom or O atom of the CO molecule, or in a p-bonded geometry via the CRO bond. These three interactions show nCO frequencies which are respectively strongly blue shifted, weakly red shifted or relatively unshifted with respect to the CO monomer. The size of the shift is roughly proportional to the strength of the interaction. These trends are consistently observed for surface adsorption, gas phase interactions and matrix isolated complexes. Surface adsorption of CO shows general characteristics that are in some respects intermediate between gas phase


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interactions and matrix isolated interactions. For gas phase complexes of CO with a second species is generally able to adopt the global minimum energy configuration, whereas with the confinements of matrix isolated complexes, multiple configurations are often observed. Multiple adsorption configurations are apparent for CO adsorbed on several of the surfaces we have studied. However, there is no evidence for any OCO-bonded molecules adsorbed on any of the surfaces, despite their existence in matrix isolation experiments and computational predictions of their stability. Therefore, the CO molecule has some ability to reorientate on the surface of these films during adsorption. In addition to the well known blue shifted feature in the doublet profile of CO adsorbed on water, we also observe a blue shifted nCO feature for CCO bonded molecules at CO and CO2 surfaces. No such features are observed for CO on NH3 and CH3OH surfaces. Gas phase experiments and computations suggest that while on the NH3 surface the CCO bonded complex is stable but not energetically favoured, a strongly blue shifted nCO feature is expected for CO on an CH3OH surface. Its absence therefore supports the conclusion that the dangling OH bonds necessary for such an adsorption configuration are not present at the surface of the CH3OH film.

Acknowledgements We gratefully thank Professor Maciej Gutowski and his research group for helpful discussions. This research was funded by the Engineering and Physical Sciences Research Council (EPSRC) U.K., and the LASSIE Initial Training Network, which is supported by the European Commission’s 7th Framework Programme under Grant Agreement No. 238258. JWD acknowledges the support of a STFC Studentship.

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