CHEMPHYSCHEM ARTICLES DOI: 10.1002/cphc.201300670

Molecular-Level Understanding of Ground- and ExcitedState OH···O Hydrogen Bonding Involving the Tyrosine Side Chain: A Combined High-Resolution Laser Spectroscopy and Quantum Chemistry Study Himansu S. Biswal,*[a] Surjendu Bhattacharyya,[b] and Sanjay Wategaonkar[b] The present study combines both laser spectroscopy and ab initio calculations to investigate the intermolecular OH···O hydrogen bonding of complexes of the tyrosine side chain model chromophore compounds phenol (PH) and para-cresol (pCR) with H2O, MeOH, PH and pCR in the ground (S0) state as well as in the electronic excited (S1) state. All the experimental and computational findings suggest that the H-bond strength increases in the S1 state and irrespective of the hydrogen bond acceptor used, the dispersion energy contribution to the total interaction energy is about 10–15 % higher in the S1 state compared to that in the S0 state. The alkyl-substituted (methyl; + I effect) H-bond acceptor forms a significantly stronger H bond both in the S0 and the S1 state compared to H2O, whereas the

aryl-substituted (phenyl; R effect) H-bond donor shows a minute change in energy compared to H2O. The theoretical study emphasizes the significant role of the dispersive interactions in the case of the pCR and PH dimers, in particular the CH···O and the CH···p interactions between the donor and acceptor subunits in controlling the structure and the energetics of the aromatic dimers. The aromatic dimers do not follow the acid–base formalism, which states that the stronger the base, the more red-shifted is the XH stretching frequency, and consequently the stronger is the H-bond strength. This is due to the significant contribution of the dispersion interaction to the total binding energy of these compounds.

1. Introduction Hydrogen bonding interactions have been widely studied owing to their immense importance in governing several physical properties of a broad variety of substances, solute–solvent interactions, crystal structures, and more importantly their role in controlling, potentially even determining the stability and activity of biological systems.[1] The influence of hydrogen bonds in molecular and supramolecular photochemistry and photobiology is also a well-recognized fact.[2] This interesting and complicated phenomenon has been extensively studied by a variety of experimental and theoretical methods.[3] Among the experimental techniques, laser spectroscopic methods are the prominent ones for exploring both inter and intra[a] Dr. H. S. Biswal School of Chemical Sciences National Institute of Science Education and Research Institute of Physics Campus Sachivalaya Marg, PO: Sainik School Bhubaneswar - 751 005 (India) Tel: ( + 91) 674-2306591 E-mail: [email protected] [b] S. Bhattacharyya, Prof. S. Wategaonkar Department of Chemical Sciences Tata Institute of Fundamental Research Homi Bhabha Road, Colaba Mumbai 400 005 (India) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201300670.

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molecular H-bonded systems at the molecular level.[4] In most cases IR/UV double-resonance spectroscopic methods have been exploited to extract the infrared signatures of H-bonded complexes. However, most of these experiments have only been performed for the electronic ground state of the corresponding H-bonded complexes. The effect of hydrogen bonding in the electronic excited state cannot be neglected. It plays a crucial role in fluorescence quenching, photo-induced electron transfer (PET), excited-state intramolecular proton transfer (ESIPT), and vibrational energy relaxation (VER).[5] However, there is a dearth of both experimental and theoretical studies in the electronic excited state hydrogen bonding at the molecular level. It is mainly due to the fact that the extremely short time scales involved in excited-state H-bonding dynamics force experimentalist to adopt highly sophisticated ultrafast spectroscopic methods. In contrast, high computational costs and the implementation of accurate theoretical methods restrict theoreticians from studying excited-state H-bonded complexes as extensively as those in the ground state. There are only few computational studies on intermolecular hydrogen bonding in the electronic excited state,[6] for which the time-dependent density functional theory (TD-DFT) has been extensively used. The lack of systematic and comparative studies on the structure, energetics, and the nature of hydrogen bonds in the electronic excited state prompted us to study these systems in ChemPhysChem 2013, 14, 4165 – 4176

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CHEMPHYSCHEM ARTICLES a comprehensive manner. Phenol-based chromophores are generally used as prototypes to study the solute–solvent interactions in complexes, both in the ground state and the electronic excited state.[7] Among these, para-cresol (pCR) has been found be an ideal chromophore to model the tyrosine side chain, as well as many biomolecules, such as photoactive yellow proteins (PYP).[8] In this work experiments were carried out using pCR as the model compound for the side chain of tyrosine, which acts both as an H-bond donor and H-bond acceptor, whereas H2O and methanol (MeOH) were taken as Hbond acceptors. The data obtained on pCR in this work was complemented by the experimental data on phenol (PH) and its complexes available in the literature.[4] Computations were carried out on both the pCR as well as the PH complexes to ensure that the comparison can be done at the same level of theory. In addition, the inductive and resonance (+ I effect and R effect) on the ground and the excited H-bond properties were explored, by choosing appropriate H-bond acceptors, namely H2O, MeOH, pCR, and PH. We combined both experimental and computational methods to extract molecular-level information about these Hbonded systems in the electronic excited state and the ground state. High-resolution laser spectroscopic techniques, that is, laser-induced fluorescence (LIF), dispersed fluorescence (DF), two-color resonant two-photon ionization (2c-R2PI), time-offlight mass spectrometry (TOFMS), resonant ionization-detected infrared spectroscopy (RIDIRS), and fluorescence-detected infra-red spectroscopy (FDIRS) were used to obtain the spectroscopic signatures. All experiments were carried out in the gas phase under supersonic jet conditions. This allowed us to study the isolated 1:1 H-bonded complexes free from the surrounding solvation and/or neighboring group effects. Computational methods such as the second-order Møller–Plesset perturbation theory (MP2) and the second-order approximated coupled cluster (CC2) model within the resolution-of-the-identity (RI) approximation were used for structure determination and binding energy calculations. The atoms-in-molecules (AIM) and natural bond orbital (NBO) theories were employed to characterize the H-bond interactions both in the ground state (S0) and electronic excited state (S1). This paper is organized as follows: after an overview of the experimental (Experimental Details) and theoretical techniques (Theoretical Details) employed, the UV and IR/UV spectra of the pCR·L (L = H2O, MeOH, and pCR) complexes are presented and compared with similar H-bonded systems (Section 2.1 and 2.2). The computed equilibrium structures and binding energies of the PH·L and pCR·L complexes for the states S0 and S1 are presented in Sections 2.3 and 2.4. The AIM and NBO analysises, which corroborate the experimental findings, are discussed in Sections 2.5 and 2.6. The structural changes of the complexes involved by going from S0 to S1 and the consequences thereof are presented in Section 3.1, followed by the discussion of spectroscopic evidences, which substantiate the theoretical results (Section 3.2). The contribution of dispersion energy to the ground as well as in the excited state is outlined in Section 3.3, and finally, the two experimental H-bond descriptors (D0 and nOH) are compared in Section 3.4.  2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemphyschem.org Experimental Details The electronic spectra for the monomer (pCR) and the complexes were obtained by two-color resonant two-photon ionization (2cR2PI) time-of-flight mass spectrometry (TOFMS) and laser-induced fluorescence (LIF). The experimental setup for the 2c-R2PI–TOFMS has been described in detail previously.[9] The complexes of pCR with water or methanol were formed by supersonic jet expansion through a 500 mm-pulsed nozzle (General Valve, series 9) of a gas mixture containing vapors of pCR obtained at 50–60 8C and 0.5– 1 % of water or methanol in helium. The typical backing pressure employed during the experiments was 2.5–3 atm. The working pressure in the source chamber was ~ 6  105 Torr and in the TOFMS chamber it was ~ 2  106 Torr. For the 2c-R2PI experiments, two 10 Hz, nanosecond Nd3 + :YAG (Quantel Brilliant) pumped dye lasers were used; a Quantel TDL70 dye laser was used for the tunable S1–S0 excitation source, while a Molectron DL18P dye laser was used to provide the fixed ionization source (D0–S1). The co-propagating excitation and ionization laser overlapped spatially and temporally and were focused onto the molecular beam using a 50 cm focal length lens. Typical pulse energies were ~ 5–10 mJ for the excitation laser and ~ 100 mJ for the ionization laser. The ions were collected by a 25 mm diameter channeltron multiplier (Dr. Sjuts Optotechnik GmbH; KBL25RS). The output of the channeltron was sent to a digitizing storage oscilloscope (LeCroy 9450) interfaced to a PC through a preamplifier (ORTEC, Model VT120) and processed through a Lab View-based computer program. The LIF spectroscopy was carried out in the expansion chamber itself. The total fluorescence was collected perpendicular to the plane defined by the laser and the molecular beam using a 50 mm diameter lens with f1 aperture and focused onto a 1P28 photomultiplier by a 100 mm focal length lens. The laser line scatter was filtered using a WG-320 long pass filter. The Dispersed fluorescence (DF) or single vibronic level fluorescence (SVLF) spectroscopy was carried out using a PMT (Hamamatsu R943)/monochromator (McPherson Inc., Model 2035) assembly. The total fluorescence collected was dispersed thorough a 35 cm monochromator with a 1200 grooves mm1 grating. The slit width was typically set to 100–200 mm corresponding to a band pass of 15 cm1. For the IR spectroscopy, IR/UV double-resonance techniques such as the resonant ion-detected infrared spectroscopy (RIDIRS) and fluorescence-detected infrared spectroscopy (FDIRS) were used. The details of these techniques have been given elsewhere.[10] The IR spectra were recorded by monitoring the depletion of either the ion signal (RIDIRS) or the fluorescence signal (FDIRS) as a function of IR frequency between 3300–3750 cm1 corresponding to the O H stretching frequency region. The tunable IR source was generated by the difference frequency generation method. In this case the dye laser output (750–790 nm) was mixed with the 1064 nm output of the Nd3 + :YAG laser in a LiNbO3 crystal to generate the IR spectra in the OH stretching frequency region. The dye laser was a ~ 10 ns, 10 Hz-seeded Nd3 + :YAG (Quanta-Ray PRO Series, PRO 230-10) pumped dye laser (Sirah, CSTR LG 18 532) The UV and the IR lasers were temporally synchronized by a master controller (SRS DG-535).

Computational Details The ground-state geometry optimization for the monomer and the complexes was done at the Møller–Plesset level of theory (MP2) as well as the second-order approximated coupled cluster (CC2) model within the resolution-of-the-identity (RI) approximation.[11] ChemPhysChem 2013, 14, 4165 – 4176

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CHEMPHYSCHEM ARTICLES The geometry optimization was carried out applying both the standard gradient and the counter-poise (CP) gradient using the method described by Boys and Bernardi.[12] For the MP2 calculation Pople’s triple z basis set with polarization functions [6-311G(d,p)][13] and for the RICC2 calculation Achlrich’s valance triple z with one set of polarization functions per atom (TZVP)[14] was used. The excited-state calculations were performed at the RICC2/TZVP level of theory. The equilibrium structures were examined by the harmonic vibrational frequency calculations to ensure that they were true minima. The interaction energies for all the complexes were corrected for the zero point energies (ZPE), the basis set superposition error (BSSE), and the fragment relaxation energies. A scaling factor of 0.9609 was used for the vibrational frequencies. This scaling factor was obtained by matching the experimental ground-state OH stretching frequency of phenol (3657 cm1) with that of the computational one (3806 cm1). The reliability of the computational methods in predicting OH stretching frequencies and the mentioned scaling factor was tested by plotting the scaled frequencies versus the experimental values. The graph is provided in the Supporting Information (Figure S1). It shows a linear correlation between the scaled frequencies and the experimental ones. All the MP2 calculations were carried out using the Gaussian 03 program suite,[15] while all the RICC2 calculations were accomplished with the TURBOMOLE program package (Version 6.4).[16] The AIM[17] theory was used to investigate the topology of the electron densities and the intermolecular hydrogen-bonding interactions. Topological properties of the electron densities for the monomers and the complexes were calculated using the AIM2000 program.[18] The wavefunctions computed at the RICC2/TZVP level using the structures were used to calculate the electron density 1(r) and the Laplacian !21(r) at the bond critical points (BCPs). To evaluate the direction and magnitude of the donor–acceptor interactions NBO[19] analysis was performed for all the complexes using the NBO 5.0 program.[20] The NBO analysis was done at the HF/ TZVP level for the optimized structures obtained at the RICC2(CP)/ TZVP level of theory.

2. Results and Analysis 2.1. Electronic Spectroscopy

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Figure 1. 2c- R2PI spectra of a) pCR,[8b] b) pCR·H2O,[8b] c) pCR·MeOH, and d) the pCR dimer. The ionization laser was set at 30 770 cm1.

sponding value of the monomer are listed in Table 1 along with the relevant data for the PH·L (L = H2O, MeOH, and PH) complexes.[4, 23] The magnitudes of band-origin redshifts in the present case are similar to those of the PH·L complexes.[23] The similarity of the spectral features and the comparable redshifts of PH·L and pCR·L complexes suggests that both PH and pCR form the same type of H-bonded complexes with the aforementioned H-bond acceptors. The REMPI spectrum of pCR·H2O was the simplest one of the three complexes investigated; with the increase in size of the H-bond acceptor the spectra became richer in that the number of low-frequency transitions increased. Figure 2 shows an expanded view of the 2c-R2PI spectra of pCR·H2O (Figure 2 a), pCR·MeOH (Figure 2 b), and pCR·pCR (Figure 2 c) up to 250 cm1 towards the blue side of their corresponding band origins. For the pCR·MeOH complex, a three-member progression with an equal spacing of 25 cm1 was observed at the band origin as well as at a transition at 31 cm1. The pCR·pCR shows a long Frank–Condon progression for a low-frequency (~ 12 cm1) bending mode at the band origin, and also transitions at 8 and 108 cm1. The transitions at 155 cm1 in Figure 2 a, 172 cm1 in Figure 2 b, and 108 cm1 in Figure 2 c were assigned as the intermolecular stretching frequency (s1) of the

The 2c-R2PI spectra of pCR (Figure 1 a), pCR·H2O (Figure 1 b), pCR·MeOH (Figure 1 c), and pCR·pCR (Figure 1 d) were recorded in the region of the S0–S1 transitions of the pCR monomer and are shown in Figure 1. In all cases the ionization laser energy was set to 30 770 cm1, that is, just above the D0–S1 transition of pCR.[21] The lowest energy transitions of pCR,[8b] pCR·H2O,[8b] Table 1. Position of the 0–0 transition (band origin), the band-origin shift (DE), the ground-state (s’’) and the pCR·MeOH, and pCR·pCR were excited-state intermolecular stretches (s’), and their corresponding force constants. observed at 35 331, 34 974, DE s’’ k’’s s’ k’s k’s/k“s Molecules BO 34 906, and 35 005 cm1, respec[cm1] [cm1] [N m1] [cm1] [N m1] [cm1] tively. These transitions were asPH 36 348[a] 0 – – – – – signed as the 0–0 transitions or [a] [a] [a] O 35 993 355 150 20.0 156 21.7 1.08 PH·H 2 the band origins of the respec415 162[a] 36.9 175[a] 43.1 1.17 PH·MeOH 35 933[a] tive species. The redshifts in the 304 110[b] 33.5 119[b] 39.2 1.17 PH·PH 36 044[b] band origin of pCR·H2O, pCR 35 331 0 – – – – – 34 974 357 148 19.9 155 21.8 1.10 pCR·H2O pCR·MeOH, and pCR·pCR compCR·MeOH 34 906 425 160 37.2 172 43.0 1.16 plexes were 357, 425, and pCR·pCR 35 005 326 102 33.1 108 37.1 1.12 326 cm1, respectively. The [a] Ref. [23b]. [b] Ref. [27]. DE: band-origin (BO) shift of the complexes with respect to the monomer PH and band-origin redshifts of the compCR. plexes with respect to the corre 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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www.chemphyschem.org the S1 state and in the S0 state for all the complexes are provided in Table 1. For all the complexes the s1 values in the S1 state are higher relative to those in the S0 state. This suggests that the hydrogen bonding is stronger in the excited state.

2.2. Infrared Spectroscopy The IR spectra in the region of the OH stretching frequency were recorded using RIDIR spectroscopy. The wavelength region between 3250 and 3750 cm1 was scanned by the IR laser (pump laser) with the UV laser (probe laser) set on the band-origin transition of the corresponding species. In all the cases, the IR laser was introduced ~ 50 ns ahead of the UV laser. The RIDIR spectra of pCR (Figure 4 a), pCR·H2O (FigFigure 2. 2c-R2PI spectra of a) pCR·H2O,[8b] b) pCR·MeOH, and c) the pCR dimer with respect to their corresponding band origins observed at 34 974, 34 906, and 35 005 cm1, respectively.

pCR·H2O, pCR·MeOH, and pCR·pCR complexes, respectively. The intermolecular stretching frequency, s, gives information about the force constant involved, which can be used as a direct measure of the strength of the hydrogen bond (vide infra). To obtain the ground-state spectral information, especially the magnitude of the intermolecular stretching frequency, s1, dispersed fluorescence (DF) or single vibronic level fluorescence (SVLF) spectra were recorded for both the monomer and the complexes by exciting their corresponding band origins. Figure 3 shows the SVLF spectra of pCR[8b] (Figure 3 a), pCR·H2O[8b] (Figure 3 b), pCR·MeOH (Figure 3 c), and pCR·pCR (Figure 3 d) up to 1050 cm1 from the resonance transition. Apart from the intramolecular transitions due to the most active modes 6a1 and 11, new transitions were observed at 148, 160, and 102 cm1 for the pCR·H2O, pCR·MeOH, and pCR·pCR complexes, respectively. These were assigned as the intermolecular stretching (s1) frequencies. The two intramolecular modes are not affected by the intermolecular hydrogen bonding (Figure 3). The intermolecular stretching frequencies (s1) in

Figure 3. SVLF spectra of a) pCR·H2O,[8b] b) pCR·MeOH, and the pCR dimer for the band-origin excitations.

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Figure 4. RIDIR spectra of the S0 state of a) pCR,[8b] b) pCR·H2O,[8b] c) pCR·MeOH, and d) the pCR dimer recorded while tuning the probe laser at the band origin of the respective species.

ure 4 b), pCR·MeOH (Figure 4 c), and pCR·pCR (Figure 4 d) are shown in Figure 4. The OH stretching frequencies of pCR, pCR·H2O, pCR·MeOH, and pCR·pCR in the S0 state were observed at 3658,[8b] 3531,[8b] 3467, and 3534 cm1, respectively.[23] The dips at 3681 and 3658 cm1 in Figure 4 c, d correspond to the free OH stretching frequencies of the acceptors MeOH and pCR. The OH stretching frequencies for the monomers (pCR and PH) as well as their complexes are listed in Table 2. Compared to the monomer the OH stretching frequency redshifts are 127, 191, and 124 cm1 for the pCR·H2O, pCR·MeOH, and pCR·pCR complexes, respectively. The OH stretching frequency redshifts for the PH·H2O, PH·MeOH, and PH·PH complexes[23] are of similar magnitude. The IR spectra in the S1 state were recorded using FDIR spectroscopy. In this case the IR laser (pump) was introduced just after (~ 5 ns) the probe laser. The pump-probe delay was adjusted to observe both the ground state and the excited state FDIR simultaneously. Figure 5 shows the excited-state FDIR spectra of pCR·H2O[8b] (Figure 5 a), pCR·MeOH (Figure 5 b), and pCR·pCR (Figure 5 c). The excited-state OH stretching frequencies for the pCR·H2O, pCR·MeOH, and pCR·pCR complexes were ChemPhysChem 2013, 14, 4165 – 4176

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Table 2. Ground-state and excited-state OH stretching frequencies and corresponding redshifts of the OH stretching frequencies with respect to the monomers. Molecules

n“OH [cm1]

Dn”OH [cm1]

n’OH [cm1]

Dn’OH [cm1]

PH PH·H2O PH·MeOH PH·PH pCR pCR·H2O pCR·MeOH pCR·pCR

3657[a] 3524[c] 3456[c] 3530[d] 3658 3531 3467 3534

– 133 201 127 – 127 191 124

3581[b] 3388[c] 3281[c] – 3581 3393 3293 3376

– -193 -300 – – 188 288 205

[a] Ref. [4b]. [b] Ref. [25]. [c] Ref. [4k]. [d] Ref. [10c]

Figure 5. FDIR spectra of the S1 state of a) pCR·H2O,[8b] b) pCR·MeOH, and c) the pCR dimer recorded while tuning the probe laser at the band origin of the respective species. The pump laser was introduced ~ 5 ns after the probe laser.

observed at 3393, 3293, and 3376 cm1, respectively. The FDIR spectrum of the pCR dimer shows a progression of a ~ 10  1 cm1 vibronically coupled band on the excited-state OH stretching mode. This progression could be attributed to the low-frequency bending vibration which was also observed in the 2c-R2PI spectrum. We could not observe the excited-state OH stretching of the monomer (pCR) because of its short fluorescence lifetime. Therefore, for the OH stretching frequency of pCR in the S1 state we assumed the same value as the one known for PH (3581 cm1)[24, 25] due to the similarity of these two compounds in many respects. The excited-state O H stretching frequencies of the pCR·H2O, pCR·MeOH, and pCR·pCR complexes were compared to the value of the monomer (3581 cm1).[23, 25, 26] The excited-state OH frequency redshifts are thus 188, 288, and 205 cm1 for the pCR·H2O, pCR·MeOH, and pCR·pCR complexes, respectively; they are much larger than their corresponding ground-state values of 127, 191, and 124 cm1. The larger redshifts of the OH stretching frequencies in the excited state suggest that all the ligands investigated form stronger complexes in the excited state than in the ground state.  2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

2.3. Equilibrium Geometry The geometry optimizations for the monomers and the complexes for the electronic ground state were done at the MP2/ 6-311G(d,p) and RICC2/TZVP level of theory using both the standard gradient and counter-poise gradient procedure. These two levels of theory were chosen because of the fact that both levels account for the dispersion interaction. The dispersion interaction plays a vital role in determining the structure and stabilization energy of the pCR dimer and the PH dimer, to which the p···p and CH···p interactions contribute significantly. Moreover recently has been demonstrated by Kleinermanns et al.[27] that the PH dimer structures obtained at these two levels of theory match well with the experimentally determined structure. In addition it was observed that the structural parameters of the complexes obtained using RICC2CP/TZVP level match well with the experimental values (vide infra). Here onwards the computed results of the RICC2-CP/ TZVP level of theory are presented. The structural parameters used to define the hydrogenbonded complexes are shown in Figure 6. The intermolecular geometrical parameters such as the hydrogen-bond distance (d = H1DOA), the distance between the oxygen atom of the donor (pCR or PH) and the acceptor (R = ODOA), the distance between the center of mass (c.m.) of the donor and acceptor (Rc.m.), the H-bond angle (q = ODH1DOA), and the intermolecular tilt angle (a = C1DODOAC1A) for the S0 and S1 state are listed in Table 3. The H-bond distance in the electronic excited state in all the cases investigated is decreased compared to that in the ground state. Irrespective of the H-bond acceptor, the H-bond distance is approximately 8 pm shorter in the electronic excited state. This is consistent with the fact that phenols and substituted phenols are much stronger acids in the

Figure 6. RICC2-CP/TZVP optimized structure of the pCR dimer. The geometrical parameters such as the hydrogen bond distance (d), the distance between the O atom of pCR and the acceptor atom (R), and the H-bond angle (q) are also depicted in the figure. D, A, and CM stand for the H-bond donor, H-bond acceptor, and center of mass, respectively.

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the H-bond donor is electronically excited, whereas the H-bond acceptor is in the S0 state. In Molecules RICC2 (CP Gradient) RICC2 (CP Gradient) fact, for the hydrogen-bonded S0 State S1 State complexes of PH and substituted q jaj r R RCM q jaj r R RCM phenols it is reported that the PH·H2O 193.9 291.0 443.3 174.3 – 185.7 283.3 440.9 170.9 – electronic excitation is solely loPH·MeOH 188.6 285.1 440.6 169.1 77.5 180.6 278.5 441.5 171.1 73.9 calized on the donor moiety.[27, 29] PH·PH 195.6 291.9 527.7 170.0 60.7 187.7 284.9 529.9 169.7 37.6 Thus, the binding energy of the pCR·H2O 194.3 291.4 481.7 174.2 – 186.1 283.6 479.6 170.8 – pCR·MeOH 189.1 285.5 472.4 168.6 75.5 180.9 278.8 475.4 170.8 74.0 dimer in the S1 state was calcupCR·pCR 195.8 292.2 568.2 170.3 54.0 187.5 284.6 573.3 169.5 32.0 lated by taking the energy of The distances r, R, and RCM are given in pm, and the angle (q) and the dihedral angle (a) are given in degree. the S1 state of the donor and See Figure 6 for the definition of r, R, RCM, q, and a. the energy of the S0 state of the acceptor in the S1-state geometry of the hydrogen-bonded excited state,[28] which is caused by the substantial strengthencomplex. The computed binding energies for the PH·H2O, ing of the H-bond interactions in the excited state. The interPH·MeOH, and PH·PH complexes in the ground state are 5.14, molecular tilt angle decreases from the trans-linear H2O com6.28, and 5.52 kcal mol1, respectively. For the former two complexes to the folded (a  55  58) aromatic dimers in the plexes the computed values are within the 90 % of the experiground state. This is due to increased dispersion interactions mentally determined values. Similar magnitudes of binding ensuch as CH···O and CH···p hydrogen bonds from H2O ergies were also obtained for the pCR complexes. In the excitMeOHPH/pCR H bond acceptors (vide infra). It was also obed state the binding energy increases. The excited-state bindserved for the aromatic dimers that going from the S0 to the S1 ing energies for pCR·H2O, pCR·MeOH, and pCR·pCR complexes are 5.67, 7.05, and 6.42 kcal mol1, respectively. From Table 4 it state, the intermolecular tilt angle, a, decreases from 55  58 to 30  58 and the dihedral angle between the acceptor ring can also be seen that, irrespective of the donors the binding plane and the plane containing the hydrogen bond changes energies for all the complexes in the S1 state are higher by about 12–18 % compared to those in the S0 state. The maxifrom 178 to 538(b=C2DC1DODOA). The given values correspond to the pCR dimer. Similar changes were also noticed for mum increase was observed for the MeOH complexes. The the PH dimer[27] as well as the hydroquinone dimer.[9] The sigcomputed values also indicate that the alkyl substitution at the H-bond acceptor site enhances both S0 and S1 state H-bond nificant changes in these two dihedral angles affect the 2cR2PI spectrum of the pCR dimer (vide infra). strength compared to the aryl substitution (normalized with respect to H2O). Table 3. RICC2-CP/TZVP-computed structural parameters of all the H-bonded complexes in the S0 and the S1 state.

2.4. Binding Energy

2.5. Atoms in Molecules (AIM) Study

The binding energies for the all the complexes both in the S0 and S1 state are listed in Table 4. The values are corrected for the basis set superposition error (DEBSSE), the deformation or relaxation energy (DERelax), and the zero point energy difference (DZPE). The DZPE correction was done using the frequencies computed at the RICC2/TZVP level of theory using 0.9609 as the scaling factor for the vibrational frequencies. For the calculation of the binding energy in the S1 state, it is assumed that

Table 4. Ground-state and electronic-excited state binding energies for all the complexes, calculated at the RICC2-CP/TZVP level of theory. Molecules

S0 State Binding Energy D0[b] D Expt[a]

S1 State Binding Energy Expt[a] D0[b] D

PH·H2O PH·MeOH PH·PH pCR·H2O pCR·MeOH pCR·pCR

5.60 6.11

6.62 7.30

5.14 6.28 5.52 5.05 6.17 5.67

8 3

5.99 7.41 6.38 5.67 7.05 6.42

10 2

The binding energy is given in kcal mol1 and the error is given in %. [a] Ref. [36]. [b] The computed binding energies are corrected for the ZPE, BSSE, and fragment relaxation.

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AIM[17] calculations were done using the ab initio wavefunctions computed at RICC2/TZVP level of theory for the monomers and the complexes. The AIM criteria proposed by Popelier[30, 31] for a classical hydrogen bond were applied for the hydrogen-bonded complexes in the S0 and S1 state. The topological parameters, such as the charge density [1H···O], and the Laplacian of the charge density [ !21H···O] determined at the BCPs, and the hydrogen bond radii (i.e. the distance between the BCP and the H atom, RH···BCP) are listed in Table 5. Figure 7 a and b show the BCPs along the lines joining the OH and O atom for the PH dimer in the S0 and S1 state, respectively, suggesting the presence of strong OH···O hydrogen bond between the two PH monomers. Similar molecular graphs were also constructed for the H2O, MeOH complexes with PH and pCR and the pCR dimer (not shown here). The charge densities at the BCPs were in the ranges of 0.0227–0.0284 au and 0.0283–0.0339 au, whereas the Laplacians for the chare densities were 0.0875–0.0993 and 0.0982– 0.1095 au for the S0 and S1 state H-bonded complexes, respectively. These values of electron density and its Laplacian are well within the range specified for the existence of hydrogen bonds (0.002–0.040 and 0.024–0.139 au, respectively).[30, 32] The ChemPhysChem 2013, 14, 4165 – 4176

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10.6, and 5.7 kcal mol1 for the H2O, MeOH, and aromatic dimers, respectively. It was also ð2 Þ Molecules S0 State S1 State noticed that Ei!j* is maximum !21H···O !21H···O 1H···O RH···BCP 1H···O RH···BCP for the MeOH complexes and PH·H2O 0.0241 0.0911 0.6678 0.0293 0.1031 0.6271 minimum for the aromatic PH·MeOH 0.0284 0.0993 0.6429 0.0339 0.1099 0.6016 dimers, suggesting that MeOH PH·PH 0.0227 0.0880 0.6840 0.0283 0.0980 0.6388 forms the stronger H-bonded 0.0239 0.0904 0.6699 0.0290 0.1031 0.6271 pCR·H2O complexes than H2O and pCR or pCR·MeOH 0.0282 0.0986 0.6454 0.0336 0.1095 0.6035 ð2 Þ pCR·pCR 0.0227 0.0875 0.6849 0.0286 0.0982 0.6373 PH. In fact the Ei!j* values for the PH and pCR dimers are about 2 kcal mol1 lower than their corresponding H2O complexes in the S0 state. This does not match well with the experimental OH frequency shift nor with the computed binding energy. It has been reported earð2Þ lier that Ei!j* values do not correlate with the XH frequency shifts and binding energies for complexes, for which dispersion interactions significantly contribFigure 7. The molecular graphs of a) the PH dimer in the S0 state, and b) the PH dimer in the S1 state obtained ute to the overall stabilization.[8] using the RICC2/TZVP wavefunctions. The bond critical points and ring critical points are represented by small Therefore, it is inferred that the dark grey and light grey balls, respectively. aromatic dimers are dominated ð2 Þ by dispersion. The larger Ei!j * values in the S1 state than the S0 state show qualitatively that charge densities and the Laplacian of the charge densities at the BCPs for the complexes in the S0 state are significantly the hydrogen bonds of the complexes are stronger in the former state. The electron population in the antibonding orbismaller than their corresponding values in the S1 state (see tal [d(s*OH)] is yet another indicator for the H-bond strength Table 5). The bond strength can be related to the magnitude of the charge density and in that sense these are consistent and the OH frequency shift. The larger the value of d(s*O-H), with the computed binding energies: the H-bond strength inthe weaker is the OH bond strength. The redshift in the OH creases in the electronic excited state. In addition, Figure 7 frequency is thus consequently larger and the hydrogen bond shows BCPs along the C2DH2D···OA and C2DH2D···C1A bond stronger. The plot between d(s*OH) and the OH frequency paths in the S0 and the S1 state, respectively. This suggests the shift (Figure 8) exhibits a good correlation, with correlation coexistence of an additional CH···O interaction in the S0 state efficient of 0.987. ð2 Þ and of a CH···p interaction in the S1 state for the PH and pCR The second-order perturbation energy, Ei!j* for the lone pair dimers. (LP) (OA) and the s*C2DH2D is 0.08 kcal mol1 in the ground state, which indicates a weak hydrogen bonding interaction between the C2DH2D and OA. In the case of the S1 state several 2.6. Natural Bond Orbital (NBO) Analysis CH···p interactions were observed. These interaction were observed between sC2DH2D and p*C1AC6A, pC1AC6A and s*C2D The NBO analysis was used to get a clear picture about the inH2D, and sC2DH2D and p*C1AC6A. The pCR dimer was chosen teraction between the donor and acceptor orbitals. The ð2Þ as a representative to depict all the inter molecular interactions donor–acceptor interaction energy (Ei!j* ) is generally estimated by the second-order perturbation theory.[19] Table 6 lists the ð2Þ 1 Table 6. Summary of the NBO analysis; Ei!j * is in kcal mol , all other values are in a.u. The values in the parenchanges in the atomic charge on thesis give the individual contributions of the non-bonded orbitals of the acceptor oxygen atom. the hydrogen [Dq(H)], the occuS1 State Molecules S0 State pancy in the antibonding orbital ð2Þ ð2Þ Dq (H) d (s*OH) Ei!j Dq (H) d (s*OH) Ei!j * * [d(s*OH)], and the second-order ð2 Þ PH·H O 0.0394 0.0136 7.70 0.0428 0.0186 11.35 2 perturbative interactions (Ei!j* ) PH·MeOH 0.0411 0.0178 10.63 (7.43 + 3.20) 0.0456 0.0236 15.39 (11.34 + 4.05) for all the H-bonded complexes PH·PH 0.0360 0.0126 5.65 0.0370 0.0185 10,03 (4.73 + 5.30) in the S0 as well as in the S1 0.0393 0.0133 7.52 0.0429 0.0182 11.11 pCR·H2O pCR·MeOH 0.0410 0.0174 10.33 0.0458 0.0232 15.07 (10.95 + 4.12) state. (7.22+3.11) The second-order interaction pCR·pCR 0.0364 0.0126 5.68 0.0378 0.0188 10.30 (4.92 + 5.38) energies for the S0 state are 7.7, Table 5. AIM topological parameters [a.u.] for all the complexes in the S0 and S1 state computed using the RICC2/TZVP wavefunctions. The hydrogen bond radius RH···BCP is given in .

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Figure 8. Correlation plot of OH frequency shifts versus occupancies in the OH antibonding orbital of the H-bond donor [d(s*O-H)].

Figure 9. Interacting donor–acceptor NBOs of the pCR dimer in the S1 state involved in the hydrogen bonding.

observed in the S1 state. The interacting donor–acceptor NBOs of the OH···O and CH···p hydrogen bonds are shown in Figure 9. The second-order donor–acceptor perturbation ð2Þ energy, Ei!j* for the C-H···p interactions is 0.30 kcal mol1. The NBO results are consistent with the AIM results an in agreement that in the case of the PH/pCR dimers the C-H···O interaction is present in the S0 state, whereas the C-H···p interaction is ð2 Þ present in the S1 state. The Ei!j* value for the CH···p interaction is higher compared to that of the C-H···O interaction. This accounts for the smaller intermolecular tilt angle (a) in the S1 state compared to the S0 state for the PH and pCR dimers.

3. Discussion 3.1. Structural Change of the Aromatic Dimers Apart from the reduction of H-bond distance and the elongation of the donor OH bond length in the excited state, major structural changes between the S0 and S1 states were observed for a and the dihedral angle between the acceptor ring plane and the plane containing the hydrogenbond (b = C2DC1DOD  2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemphyschem.org OA). For the pCR dimer a changes from 548 to 328 while b changes from 178 to 538. The same observations were made earlier for the PH dimer[27, 33] as well as the hydroquinone dimer.[34] In the aromatic dimers, the smaller value of a in the electronic ground state compared to that of the pCR·H2O and p-CR·MeOH complexes is because of the C2DH2D···OA hydrogen bonding, which is absent in case of the H2O and MeOH complexes. The significant changes for a and b in the aromatic dimers can be rationalized as follows: In the electronic ground state one of the electron lone pairs of the acceptor oxygen is involved in the ODH1D···OA hydrogen bond, while the other electron lone pair is involved in the C2DH2D···OA hydrogen bond. In the electronic excited state both electron lone pairs of the acceptor oxygen are involved in the ODH1D···OA hydrogen bond and the distance between H1D···OA decreases up to 70 pm compared to that in the ground state. Both facts are confirmed by NBO analysis. The closest distance between an acceptor and a donor moiety in the electronic excited state was found for the C2DH2D···p interaction. The distance between H2D and C1A is 280 pm in the excited state, which is 45 pm smaller than that in the ground state. Similarly the Hbond angle (angle C2DH2D···C1A) in the S0 state (1338) is smaller compared to that in the S1 state (1528). This suggests that the C2DH2D···p interaction is stronger in the S1 state compared to that in the S0 state and also explains why the PH and pCR dimers are more folded (smaller a) in the electronic excited state than in the ground state. AIM calculations are also consistent with the conclusions derived from the geometrical parameters. Figure 7 shows the BCPs along the C2DH2D···OA and C2DH2D···C1A bond paths in the S0 state and the S1 state, respectively. The existence of an additional CH···O interaction in the S0 state and a C H···p interaction in the S1 state is suggested for the PH and pCR dimers. These additional interactions are responsible for the more folded structures of the aromatic dimers compared to the pCR PH·H2O or pCR PH·MeOH complexes. The values of the charge densities at the BCPs along the C2DH2D···OA and C2D H2D··· C1A bond paths are 0.0059 and 0.0065 au, respectively. The higher value of the charge density for the C2DH2D···C1A interaction than the C2DH2D···OA interaction suggests that the CH···p interaction in the S1 state is slightly stronger than the CH···O interaction in the S0 state.

3.2. Spectroscopic Information from UV and IR Spectroscopy 3.2.1. Shift of the Electronic Transition From the 2c-R2PI spectra it was found that the band origins of all the complexes of pCR are red-shifted with respect to the monomer band origin. The redshifts for the pCR·H2O, pCR·MeOH, and pCR·pCR complexes are 357, 425, and 326 cm1, respectively. The magnitude of these redshifts is typical of the H-bonded complexes of phenol-based chromophores and suggests that the complexes are more stable in the excited state relative to the ground state. In all cases the phenolic OH is the H-bond donor. Therefore it was concluded ChemPhysChem 2013, 14, 4165 – 4176

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CHEMPHYSCHEM ARTICLES that in the present case pCR acts as the H-bond donor, which is in line with the most stable structures obtained computationally for these complexes. For the pCR·H2O and pCR·MeOH complexes, the band-origin transition is the p–p* transition centered on the pCR monomer, the H-bond donor. However, it is not so straightforward for the pCR dimer, as the one pCR unit acts now as the H-bond donor, whereas the other unit acts as the H-acceptor. This means that the p–p* transition can be centered on either of the two units. It is useful to compare this system with other aromatic H-bonded systems. In case of the phenol dimer, the transition centered on the donor part is red-shifted by 304 cm1, whereas that one located on the acceptor is blue-shifted by 357 cm1.[35] Similar magnitudes of red- (311 cm1) and blueshifts (390 cm1) have been reported for the donor and acceptor moieties of the hydroquinone dimer, respectively.[34] This comparison leads to the conclusion that the lowest energy or band-origin transition is localized on the H-bond donor unit of the pCR dimer. The magnitude of the band-origin shift gives information about the relative stabilization of the electronic excited state compared to the ground state. The relative stabilization of the S1 state in the case of the alkyl-substituted H-bond acceptor, as in the MeOH complex (425 cm1), is much higher than its H2O (357 cm1) counterpart, whereas for the aryl-substituted H-bond acceptor the effect is opposite, that is, the redshift for the pCR dimer complex is smaller (326 cm1) than that for the H2O complex. 3.2.2. Intermolecular Vibrations The intermolecular stretching modes (s1 and s1) are very important in the context of hydrogen bonding as they run along the H-bonding coordinate. The force constants deduced from observed frequencies for these modes provide preliminary indication about the H-bond strength. Table 1 lists the force constants for the respective intermolecular stretching frequencies. The force constants were calculated from the experimental intermolecular stretching frequencies by using a crude diatomic model, in which each molecule in the complex was treated as a point mass. The trend in the increment of force constant for alkyl and aryl substitution of the H-bond acceptor was 1:1.84:1.62 (normalized with respect to H2O) in the ground state and 1:1.99:1.80 (normalized with respect to H2O) in the electronic excited state, while the ratios of the computed binding energy were 1:1.22:1.06 and 1:1.24:1.07 for the S0 and S1 state, respectively. This suggests that although the calculated force constants give qualitative information about the H-bond strength, it is unable to predict the exact trend for the H-bond donor–acceptor interaction energy. The biggest discrepancy was observed for the PH·H2O and PH·MeOH complexes. The force constants suggest that the PH·MeOH complex is about two times stronger than the PH·H2O complex, whereas the experimental binding energies of these complexes[36] indicates that the PH·MeOH complex is about 1.09 times stronger than the PH·H2O complex. This anomaly can be attributed to the mixing of intermolecular stretching modes with other low-frequency modes as observed in the case of the hydroquinone dimer. However, both force constant and binding energies sug 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemphyschem.org gest that the hydrogen bonding is stronger in the S1 state than in the S0 state for all the complexes and that the increment in the H-bond strength is little higher for the alkyl-substituted H-bond acceptor than the aryl-substituted one. 3.2.3. OH Stretching Frequency The IR data of the OH stretching frequency of the parent monomer and its shift in the complex is very useful for determining the strength and nature of interaction. From the IR redshift it can be inferred that the hydrogen bonds of the H2O complexes are weaker than those of the MeOH complexes but slightly stronger than the aromatic H-bonded complexes (pCR dimer or PH dimer). However, there is a slight problem with this picture. In the case of PH it has been shown[37] that the O H frequency redshifts are well correlated with the proton affinity (PA) of the acceptor. The PA of pCR (756 kJ mol1)[38] is almost comparable or even slightly higher than that of MeOH (754.3 kJ mol1),[39] nevertheless the OH frequency redshift in the case of the pCR dimer is considerably smaller than that in that of the pCR·MeOH complex. A similar situation is noticed for the PH·H2O, PH·MeOH, and PH·PH complexes. In these cases, the PA of PH (745 kJ mol1)[40] is much higher than that of H2O (691 kJ mol1),[39] but the OH frequency redshifts for the PH dimer and PH·H2O complex are comparable. Although the PA of PH and MeOH are comparable, the OH frequency redshift for the phenol dimer is smaller than that for the PH·MeOH complex. Therefore it must be noted that the IR shifts are not completely consistent with the acid–base formalism for H-bonding interactions. However, these observations are consistent with the fact deduced from the NBO analysis that the extent of electron transfer from the lone pair on the acceptor to the OH antibonding orbital is smaller in the case of the aromatic (pCR and PH) H-bond acceptor. Compared to the monomer the OH stretching frequency redshifts were much greater in the excited state than those in the ground state for all the complexes. This is due to the fact that in the excited state phenols become strong acids and the OH stretching frequency also red-shifts considerably. For instance, in the case of PH the OH stretching frequency shifts to 3581 from 3658 cm1 in the ground state indicating that the OH bond becomes weaker in the excited state, rendering it more labile for the proton transfer. It has been shown that the OH stretching frequency redshifts in the complexes are greater in the excited state relative to those in the ground state and that they are linearly correlating with the pKa values. It can be seen from the Table 2 that the PH complexes are slightly stronger than the corresponding pCR complexes. This is due to the fact that PH is a stronger acid than pCR both in the ground state and the electronic excited state. The pKa of PH in the S0 state is 9.98[41] and that in the S1 state is 4.0,[42] whereas the values for pCR are 10.26 and 4.3, respectively.[42] 3.3. Contribution of Dispersion Energy Components The aromatic dimer, specifically the PH dimer, is generally considered as the mixed complex in the S22 data set,[43, 44] in ChemPhysChem 2013, 14, 4165 – 4176

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persion contributions are similar in magnitude.[43] In the present context, it is interesting to see whether the correlation between Molecules S0 State S1 State the redshift of the OH-stretchRICC2 HF RICC2 HF DE %disp DE DE %disp DE ing frequencies (DnOH) and the PH·H2O 6.72 5.57 17 7.86 5.14 35 binding energy of the H-bond PH·MeOH 7.55 5.21 31 8.79 4.80 45 donor is still valid for the PH and PH·PH 6.22 2.85 54 7.28 2.29 69 6.61 5.37 19 7.59 5.24 31 pCR·H2O the pCR dimer. The plot between pCR·MeOH 7.45 4.96 33 8.50 4.91 42 DnOH and the binding energy for pCR·pCR 6.43 2.72 58 7.41 2.46 67 all the complexes is shown in [a] The interaction energies are corrected for the BSSE and fragment relaxation. [b] The dispersion or correlaFigure 10. The correlation betion energy was calculated as the difference between DERICC2 and DEHF. tween DnOH and the binding energy is poor, the correlation coefficient is 0.926. However, if which both the electrostatic and the dispersion interaction are the PH dimer and pCR dimer are removed from the set, which of the same magnitude. In the present context, it would be inthen consists of the PH·H2O, PH·MeOH, pCR·H2O, and teresting to compare the relative contributions of the disperpCR·MeOH complexes only, it can be shown that DnOH corresion energy or the correlation energy of the intermolecular hylates well with the binding energy of the complexes (Figdrogen bonding in the S0 and S1 state. The dispersion energy ure 10 b). This shows again that the general concept—the or the correlation energy was calculated as the difference bebigger the redshift of the H-bond donor XH stretching fretween the BSSE-corrected total interaction energy computed quencies, the stronger the H-bond strength—does not hold at the RICC2 level and that computed at the HF level for the equilibrium structures obtained by using the RICC2-CP/TZVP level of theory. The RICC2 energy (DERICC2), the HF energy (DEHF), and the percentage of the dispersion energy contribution (%disp) are listed in the Table 7. In the S0 state the maximum dispersion energy contribution was obtained for the aromatic (PH/pCR) H-bond acceptors, followed by MeOH and H2O. The dispersion energy of the PH dimer in the S0 state accounts to 55 % of the total interaction energy, which is similar to the values reported by Kannemann and Becke[44] using different DFT methods. The increase in dispersion interaction from H2O to MeOH to PH/pCR as H-bond acceptors correlates well with the geometry change of these complexes going from the trans-linear structure of PH·H2O to the folded form of PH dimer (vide supra). The dispersion contribution in the S1 state is higher for all the complexes compared to their corresponding values in the S0 state. Irrespective of the acceptor the increment in dispersion energy component is about 10–15 % in the S1 state. Table 7. Total interaction energy computed at the RICC2 level (DERICC2),[a] interaction energy computed at the Hartree–Fock level (DEHF),[a] and contribution percentage of dispersion interaction (%disp).[b] All the energy components are in kcal mol1.

3.4. Comparison between Dissociation Energy (D0) and IR Spectral Redshift (DnOH) of the H-Bond Descriptors The IR redshift in the donor OH group (DnOH) was conventionally taken as an indicator of the strength of the hydrogen bond. The IR redshift in the donor OH stretching frequencies correlates well with the computed binding energies of the Hbonded complexes, for which the electrostatic interactions are the major components of the stabilization energy (for the O and N H-bond acceptors). However, this does not hold well for the H-bonded complexes, for which the dispersion interactions are the major components of the stabilization energy, for example, in the case of the H-bond acceptors that involve sulfur atoms.[8, 45] The PH dimer and the pCR dimer are usually treated as the H-bonded complexes, in which the electrostatic and dis 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 10. Correlation plot of a) redshifts in OH frequencies versus binding energies (D0) for all the complexes (PH·H2O, PH·MeOH, PH dimer, pCR·H2O, pCR·MeOH, pCR dimer), b) redshifts in OH frequencies versus binding energies (D0) excluding the PH and the pCR dimer. The D0 values for the S0 and the S1 state of all the complexes were obtained at the RICC2-CP/TZVP level of theory. The OH stretching frequencies in the S0 and the S1 states for the pCR complexes are from the current work, whereas those for the PH complexes were taken from refs. [4b,j, 10c, 24].

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4. Conclusions The present paper provides both experimental and theoretical insights on the ground-and excited-state hydrogen bonding of model tyrosine side chain compounds. A direct comparison of the effects of the alkyl and aryl substitution on the hydrogen has been done at the molecular level. The alkyl-substituted Hbond acceptor enhances both the S0 and the S1 H-bond strength. In contrast, the aryl-substituted H-bond acceptor does not have a significant effect on the H-bond energy (normalized with respect to H2O as H-bond acceptor). It has also been observed that aryl-substituted H-bond acceptors lead to a folded complex, whereas alkyl-substituted H-bond acceptors increase the strength of the hydrogen bonding. The observed folded structures of the pH and pCR dimers are due to the additional CH···O and CH···p interactions in these complexes in the S0 and the S1 state, respectively. The NBO and AIM analysis also indicated that because of the presence of the CH···O and CH···p interactions in the PH and pCR dimers, they are more folded compared to their corresponding H2O and MeOH complexes. The CH···p interaction was found to be little stronger than the CH···O interaction, which accounts for the smaller intermolecular tilt angle for these dimers in the S1 state compared to the S0 state. Both the electronic spectroscopy (redshift of the band origins and the intermolecular stretches) and IR spectroscopy of the complexes indicated that the hydrogen bond is stronger in the electronic excited state than in the ground state. The increase in binding energy was found to be about 15 % in the S1 state compared to the S0 state. Both in the S0 and the S1 state the dispersion energy contribution is maximum for the PH or pCR dimers, followed by their corresponding MeOH and H2O complexes. However, for all the complexes the dispersion energy contribution was increased in the S1 state compared to their S0 state. The PH and the pCR dimer do not follow the acid–base formalism, which states that the stronger the base is (taking the same H-bond donor), the stronger is the H-bonded interaction and subsequently the larger the redshift of the OH stretching frequency. The deviation from this general rule has been observed for many cases for which the contribution of dispersion energy is significant.

Acknowledgements H.S.B. acknowledges the Department of Science and Technology (DST, Govt. of India) for the DST-Inspire faculty fellowship. This work was supported by the computational facilities of the National Institute of Science Education and Research (NISER) and the Department of Atomic Energy (DAE, Govt. of India). Keywords: FDIR · hydrogen bonds · para-cresol · phenol · REMPI · tyrosine

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