Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 1086–1098

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Molecular structure, spectroscopic properties, NLO, HOMO–LUMO and NBO analyses of 6-hydroxy-3(2H)-pyridazinone Saied M. Soliman a,⇑, Jörg Albering b, Morsy A.M. Abu-Youssef a a b

Department of Chemistry, Faculty of Science, Alexandria University, P.O. Box 426, Ibrahimia, 21321 Alexandria, Egypt Institute of Chemical Technology of Materials, Graz University of Technology, A-8010 Graz, Austria

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Molecular structure of DHP is studied

Optimized molecular structure (left), FMO (middle) and MEP (right) of the stable DHP(T1) isomer.

in gas, solution and solid phases.  The most stable isomer of DHP in all phases is the oxo-hydroxo form.  Its IR and UV spectra are predicted using DFT calculations.  Two stable H-bonded dimers have been studied.  NBO analyses were performed on the monomer and dimers.

a r t i c l e

i n f o

Article history: Received 30 July 2014 Received in revised form 25 September 2014 Accepted 30 September 2014 Available online 22 October 2014 Keywords: Tautomerism Pyridazine NBO TD-DFT H-bonding

a b s t r a c t The molecular structure and relative stabilities of the six possible isomers of 6-hydroxy-3(2H)-pyridazinone (DHP) in the gas phase and in solutions of different polarities are predicted using the B3LYP/6311++G(d,p) method. The oxo-hydroxo isomer is the most stable form in the gas phase and in solution. These results agree with our reported X-ray structure. The effect of solvents on the spectroscopic properties of the most stable isomer has been studied using the polarized continuum method (PCM) at the same level of theory. The vibrational spectra of the compound studied are calculated and compared with the experimentally measured FTIR spectra. The electronic spectra in gas phase and in solution were calculated using the TD-DFT method. The most intense absorption band is predicted at 312.4 nm and belongs mainly to a p ? p* transition. In polar solvents, this spectral band undergoes a hypsochromic shift. Two stable dimer forms were calculated at same level of theory. Dimer A is more stable than dimer B, by 6.66 kcal mol1. The former is stabilized by stronger OAH  O H-bonds compared to the weaker NAH  O interactions in the latter. The effect of these H-bonding interactions on the molecular structure and vibrational spectra of these compounds are predicted. NBO analyses were carried out to investigate the stabilization energy of various inter- and intramolecular charge transfer interactions within the systems studied. Ó 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +20 3 5917883; fax: +20 3 5932488. E-mail address: [email protected] (S.M. Soliman). http://dx.doi.org/10.1016/j.saa.2014.09.133 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

S.M. Soliman et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 1086–1098

Introduction A pyridazine (1,2-diazine) core is present in quite a number of cytostatic drugs and fungicides [1–6]. Substituted pyridazinones (hydroxypyridazines) are used in medicine as antimicrobial, fungicidal [7], antihypertensive [8–10], anti-inflammatory [11] and analgesic [12,13] agents. 6-hydroxy-3(2H)-pyridazinone (DHP) (maleic hydrazide, C4H4N2O2) is used as a selective herbicide and a plant growth regulator [14–23]. It is also used to retard the development of buds to keep them fresher for longer [24–27]. 6hydroxy-3(2H)-pyridazinone (DHP) is used as a plant-growth inhibitor [28]. The similarity in molecular structures between DHP and pyrimidine bases suggests that their erroneous incorporation into nucleic acid may be a possible mechanism of cytotoxicity [23,29–33]. It has been suggested that the molecules of DHP can act either as a purine analog (forming base pairs with uracil and thymine) or as a pyrimidine analog (forming base pairs with adenine) [33,34]. Like other nucleic acid bases, DHP shows tautomerism [35–43], such as keto-hydroxy, diketo and dihydroxy forms. The crystals of DHP have been found to exist as three different polymorphs [44–47]. In all of the polymorphs, DHP adopts exclusively the oxo-hydroxo form (Scheme 1). Tautomerism of solid maleic hydrazide was studied experimentally using infrared and Raman spectroscopies [48]. The structure of DHP in a variety of solutions was characterized experimentally using a variety of spectroscopic techniques [49–53]. All these studies have concluded that DHP, under all the experimental conditions, exists as the oxo-hydroxo isomer. However, to our best knowledge, there have been no detailed theoretical study of the spectroscopic properties of DHP and its dimers. In this work, the structure and tautomerism as well as the spectral properties of the monomeric and two dimeric DHP molecules were investigated in the frame work of density functional theory using the hybrid B3LYP method at the 6-311++G(d,p) level. The effects of H-bonding interactions on the spectral properties of the compound studied were studied using the same level of theory. Experimental details High purity 6-hydroxy-3(2H)-pyridazinone (DHP) was purchased from Aldrich Chemical Company Inc. The DHP was recrystallized from a water–methanol mixture, giving prismatic crystals suitable for X-ray measurement. The crystallographic measurements were performed using a BRUKER Kappa APEX II 4K CCD diffractometer with graphite monochromated Cu Ka radiation at 100 K. The crystallographic data, conditions used for the intensity data collection and some features of the structure refinements are listed in Table 1. Data processing, Lorentz-polarization and absorption corrections were performed using SAINT, APEX and the SADABS computer programs [54]. The structures were solved by direct methods and refined by

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full-matrix least-squares methods on F2, using the SHELXTL [55] program package. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were located from difference Fourier maps, assigned with isotropic displacement factors and included in the final refinement cycles by use of geometrical constraints in the case of hydrogen atoms bonded to carbon. The positional parameters of hydrogen atoms connected to nitrogen and oxygen atoms, involved in hydrogen bridge bond with neighboring molecules, were allowed to vary freely; their isotropic thermal parameters have been constrained to the equivalent isotropic displacement parameters of the respective oxygen and nitrogen atoms. Computational details The DHP molecule has the six possible isomers shown in scheme 1. The quantum chemical calculations were performed by applying the DFT method with the B3LYP functional and 6-311++G(d,p) basis set using Gaussian 03 software [56]. The input file was taken from the CIF file obtained from our X-ray single crystal measurements. The geometries were optimized by minimizing the energies with respect to all the geometrical parameters without imposing any molecular symmetry constraints. GaussView4.1 has been used to draw the structures of the optimized geometries [57]. All the calculations were first carried out in gas phase, then solute–solvent effects were studied by the Self-Consistent Reaction Field (SCRF) theory [58] with the Polarized Continuum Model (PCM) [59]. Frequency calculations at the optimized geometry were done to confirm the optimized structures to be at an energy minimum and to obtain the theoretical vibrational spectra. The true energy minimum at the optimized geometry of the isomers studied was confirmed by absence of any imaginary frequency modes. The total energy distribution (TED) of the vibrational modes was calculated with the VEDA program [60] and the vibrational modes were characterized by their total energy distribution (TED%). The possible electronic transitions of the most stable isomer were calculated by the TD-DFT method in different solvents to predict the effect of solvent on the electronic spectra compared to the gas phase and for visualizing HOMO and LUMO states. The natural bond orbital analyses were performed using the NBO calculations as implemented in the Gaussian 03 package [61] at the DFT/B3LYP level. Results and discussion X-ray structure The compound DHP is known to crystallize in one triclinic and two monoclinic (P21/c and P21/n) modifications as mentioned above [45]. In this work we focused on the phase crystallizing in P21/n and re-determined the structure at low temperature (100 K) in order to investigate if temperature-dependent phase transitions occur. There are only marginal differences between

Scheme 1. The structure of the suggested isomers of the 6-hydroxy-3(2H)-pyridazinone (DHP).

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Table 1 Crystal data and structure refinement for 6-hydroxy-3(2H)-pyridazinone. Empirical formula Formula weight Temperature Wavelength Crystal system Space group Unit cell dimensions

Volume Z Density (calculated) Absorption coefficient F(000) Crystal size Theta range for data collection Index ranges Reflections collected Independent reflections Completeness to theta = 27.50° Max. and min. transmission Refinement method Data/restraints/ parameters Goodness-of-fit on F2 Final R indices [I > 2sigma(I)] R indices (all data) Extinction coefficient Largest diff. peak and hole

C4H4N2O2 112.09 100(2) K 0.71073 Å Monoclinic P21/n a = 6.4652(3) Å b = 6.8980(3) Å c = 10.5125(5) Å 453.11(4) Å3 4 1.643 Mg/m3 0.135 mm1 232 0.43  0.22  0.11 mm3 3.36–27.50°

a = 90° b = 104.875(2)°

c = 90°

8 6 h 6 8, 8 6 k 6 8, 13 6 l 6 13 12,742 1038 [R(int) = 0.0295] 100.0% 0.9851 and 0.9443 Full-matrix least-squares on F2 1038/0/80 1.094 R1 = 0.0379, wR2 = 0.0991 R1 = 0.0477, wR2 = 0.1078 0.004(1) 0.396 and 0.290 e Å3

the structure determined at 300 K by Katrusiak and our results. As could be expected, the unit cell parameters decreased, the density increased and the precision of the positional and thus the interatomic distances is better at 100 K. The general features of the structure determined at 300 K are also found in the recent refinement. Figs. 1 and 2 show the numbering scheme of the DHP molecule and the packing scheme; the interatomic distances and bond angles are given in Tables 2 and 3, respectively. The asymmetric unit contains one molecule, thus the number of formula units per cell is z = 4. The molecule shows tautomerism and can have either an amide group or a hydroxy group bonded to the carbon atom in the vicinity of the pyridazine-N atoms. Both tautomeric forms occur in the same molecule in the structure of DHP: the atoms N1, H1, C4 and O2 belong to the amide group, while N2, C1, O1 and H1 are from the alpha-hydroxy imine group.

The bond lengths in the amide group are in general agreement with those found in other similar compounds (typical values: CAN 1.38 Å, C@O 1.22 Å). The C4AN1 (1.343(2) Å) bond is slightly shorter than the expected value, while the C4@O2 double bond (1.266(2)) is significantly longer than a typical carbonyl bond in amides. This finding seems to be correlated with the role of the O2 as a double proton acceptor in the hydrogen bridge network. This function obviously decreases the electron density in the C4@O2 bond and thus increases its single bond character to a certain extent. The length of the phenolic C1AO1 bond of 1.340(2) Å is comparable with the CAO distance found in e.g. phenol (1.36 Å). The bond distance of C1 to N2 is 1.301(2) Å, and thus ranges between the length of an aliphatic imine bond (1.28 Å) and the CAN distance in aromatic N-heterocyclic compound like e.g. pyridine (1.337 Å). In the solid state the DHP molecules form a two-dimensional coordination network, held together by three different kinds of hydrogen bridge bonds. Fig. 3 shows the different types of hydrogen bonds of one monomeric molecule (Fig. 3a) and the 2D-infinite network build up by the DHP monomeric units (Fig. 3b). The three hydrogen bridge bond modes are shown in different colors. Table 3 summarizes the parameters of the three different types of hydrogen bridges in the structure. Each DHP molecule is connected with a second one via two strong hydrogen bridge bonds between the amidehydrogens and the carbonyl-oxygen atoms N1AH1  O2, shown as dotted, orange lines in Fig. 2b. In the dimers there is an eight-membered ring system AN1AH1  O2@C4AN1AH1  O2@C4A, the angle between the donor atom N1, the hydrogen H1 and the acceptor atom O2 is 172.89° and the donor–acceptor distance is 2.835 Å. These dimeric units are bonded to two others by a second type of strong hydrogen bridges: the hydroxy group O1AH4 acts as donor and the oxygen atom O2 of a second dimer is the proton acceptor. Four O1AH4  O2 bridges (the dotted, green, lines in Fig. 3b) connect the dimeric units, thus forming a ladder-type polymeric unit consisting of alternating 8- and 14-membered rings. The O1AH4  O2 angle (172.38°) is very similar to the NAH  O angle in the dimers; the distance between donor and acceptor atom is significantly smaller (O1AO4: 2.602 Å) as compared with the cyclic amide dimer. A third, much weaker type of hydrogen bridges, marked in blue in Fig. 2b, occurs between the H2 atoms, bonded to a carbon atom in the ring system of the DHP molecule, and the hydroxy-oxygen atom O1, which acts as a double acceptor in this case. The donor–acceptor distance in these weak bonds is much longer than those in the more polar hydrogen bridges; the C2AO1 distance is 3.393 Å and the C2AH2  O1 angle is 174.67°. The whole network can be considered to be built-up of 6-membered (the pyridazine rings), 8-membered (the amide dimers) and 14-membered rings (the rings formed via OAH  O and CAH  O bridges, respectively. The 2D molecular networks parallel to the [1 0 0] plane are connected with another only by weak van-der-Waals contacts between the p-electrons of the carbonyl groups, the lone pairs of the hydroxyl-oxygen atoms and the p-electrons of the pyridazine ring system. The shortest intermolecular contacts perpendicular to the network plane occur between the atoms C1AO2: 3.172(3) Å, C4AO1: 3.209(3) Å and C4AC1: 3.291(3) Å.

DFT calculations

Fig. 1. Atom numbering scheme of DHP. All atom ellipsoids are shown at the 50% probability level.

Tautomerism and conformers of DHP The relative stabilities and populations of the six possible isomers of DHP molecule were calculated using the B3LYP/6311++G(d,p) method. The calculated energy predictions for the six isomers are compared in Table 4 which shows that T1 has the lowest energy (E = 414.8009 Hartrees) of the isomers studied.

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Fig. 2. Packing scheme of DHP. The molecules in the upper, right and lower, left corner of the unit cell are at located at the height. The other two molecules are separated from them by half a translation period in the projection direction.

Table 2 Bond lengths [Å] and angles [°] for 6-hydroxy-3(2H)-pyridazinone. C1AN2 C1AO1 C1AC2 C2AC3 C2AH2 C3AC4 N2AC1AO1 N2AC1AC2 O1AC1AC2 C3AC2AC1 C3AC2AH2 C1AC2AH2 C2AC3AC4 C2AC3AH3 C4AC3AH3

1.3013 (19) 1.3403 (17) 1.427 (2) 1.347 (2) 0.9500 1.441 (2) 118.64 (13) 123.96 (13) 117.40 (13) 118.62 (13) 120.7 120.7 119.45 (14) 120.3 120.3

C3AH3 C4AO2 C4AN1 N1AN2 N1AH1 O1AH4 O2AC4AN1 O2AC4AC3 N1AC4AC3 C4AN1AN2 C4AN1AH1 N2AN1AH1 C1AN2AN1 C1AO1AH4

0.9500 1.2655 (17) 1.3429 (19) 1.3712 (16) 0.863 (19) 0.912 (19) 120.28 (13) 124.18 (13) 115.55 (13) 127.16 (12) 118.0 (12) 114.8 (12) 115.22 (12) 111.5 (11)

Table 3 Hydrogen bonds for 6-hydroxy-3(2H)-pyridazinone [Å and °]. DAH

d(DAH)

d(H  A)

T6 > T2 with the others having almost zero populations. Molecular geometry The optimized structure and atom numbering scheme of the major isomer T1 are given in Fig. 5. The optimized geometric parameters such as bond lengths and bond angles obtained for this isomer using the B3LYP method with a 6-311++G(d,p) basis set are given in Table S2 (Supplementary materials). The optimized geometry is compared with the structural parameters obtained from the CIF file. The point group of the most stable isomer (T1) is C1. The N3AN4, N3AC6 and C7AO12 bonds are shorter than the experimental values while most of the other bond lengths are overestimated. The maximum deviations of the bond distances and bond angle values from the experimental data are 0.048 Å (N4AC7) and 3.102° (C1AC7AN4) respectively. In general, the bond lengths and bond angles are predicted very well. The deviations from the experimental data are attributed to the phase difference between the calculations and the experiment where the calculations refer to an isolated molecule in the gas phase (e = 1), while the experimental data are those of the molecule in the solid phase. All the calculated dihedral angles are in the range of 0.0012–0.0892° indicating that DHP(T1) is an almost planar molecule in the gas phase. The computed structural parameters in solution showed that the effect of nonpolar solvents on the structural parameters is almost insignificant. Only in polar solvents is there a noticeable change of the CAN, C@O, NAH and OAH bond distances. These bonds are of high polarity and can be involved in solute–solvent interactions with polar solvent molecules in solution [62]. Fig. 6 shows a comparison between the calculated bond distances in the gas phase and in solution. As can be seen, the C7AN4 bond is shortened while the OAH, NAH and C@O bond distances are

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Fig. 3. The three types of hydrogen bridge bonds connecting a monomeric molecule of DHP (upper part, a) with the two-dimensional infinite networks (lower part, b) in the solid state structure of this compound are shown by orange, green and blue dotted lines, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 4 Computed energies (E), energy differences (DE), Gibbs free energies (G) and population percentages of DHP isomers in the gas phase. Tautomer

E (Hartree)

DE (kcal mol1)

G (Hartree)

Pop. (%)

1 2 3 4 5 6

414.8009 414.7928 414.7916 414.7816 414.7711 414.7925

0.0000 5.0727 5.8315 12.1264 18.7116 5.2936

414.8312 414.8236 414.8218 414.8120 414.8018 414.8232

99.9413 0.0331 0.0047 0.0000 0.0000 0.0208

increased due to solvent effects in solution. The changes in these bonds are small in nonpolar solvents such as CCl4 and cyclohexane. Fig. 7 shows the optimized geometry of two dimer forms of DHP calculated at the B3LYP/6-311++G(d,p) level. The predicted point group of dimers A and B are Ci and C1 respectively. As a result,

dimer B is polar and has a dipole moment of 8.8599 Debye while dimer A is not. Dimer A presents the most stable form calculated for the DHP molecule; the dimer A is more stable than dimer B by 6.66 kcal mol1. Both intermolecular NAH  O H-bonds are equivalent in dimer A. The NAH and C@O bonds involved in the intermolecular interactions of dimer A are slightly longer than the monomer while the adjacent CAN bonds are shortened due to H-bond formation (Table S2 Supplementary materials). Similar observations are predicted for the OAH and C@O bonds of dimer B. Remarkably, in dimer B, the intermolecular H  O bond are shorter (1.747 Å) than in dimer A (1.789 Å). These results agree with the known fact that OAH  O hydrogen bonds, in general, are much stronger interactions than the NAH  O hydrogen bonds. The NAH  O and OAH  O intermolecular H-bonding interactions do not affect the planarity of the pyridazine ring. The angle between the two pyridazine ring planes is almost 0°.

Table 5 Effect of solvent on the relative energies (DE) of the DHP isomers. Tautomer

1 2 3 4 5 6

DE CCl4

Cyclohexane

DMSO

Acetonitrile

Water

Ethanol

0.0000 4.0540 6.4733 11.4173 16.6282 3.3808

0.0000 4.1842 6.4146 11.5341 16.9206 3.5938

0.0000 1.8175 7.0420 9.0025 10.9944 0.7559

0.0000 1.8426 7.0414 9.0351 10.8038 0.8158

0.0000 1.6826 6.9219 8.7933 10.6129 0.7658

0.0000 1.9639 7.0249 9.1757 11.4039 0.9301

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100.0 gas

80.0

4CCl

60.0

DMSO Acetonitrile

3T

Water

40.0

Ethanol

5T

1.1 1

Gas

Carbon cyclohexane Acetonitrile tetrachloride

DMSO

Ethanol

R(8-9)

R(10-11)

R(7-12)

R(6-8)

R(4-7)

R(4-5)

R(3-6)

R(6-10)

0.0

R(3-4)

R(1-8)

0.9

20.0

R(1-2)

6T

1.2

R(1-7)

4T

1.3

Cyclohexane

1T 2T

1.4

Water

Fig. 4. The populations of the DHP isomers in gas phase and in solution.

Natural atomic charges The natural atomic charges (NAC) at the different atomic sites of DHP(T1) calculated using the DFT/B3LYP method are collected in Table 6. From the NAC values listed in Table 6, all H-sites are electropositive where H11 is the most electropositive H-site. The nitrogen and oxygen atoms have the highest negative charge densities so; the carbon atoms bonded with these sites are electropositive. The C6 and C7 are the most electropositive C-atoms because they are lying between the two strong electronegative O and N atoms. The other carbon atoms (C1 and C8) have negative charge densities. The largest NAC variations due to the solvent effect occurs at the O10, O12, H5 and H11 atoms because these

Fig. 6. The effect of solvents on the calculated bond distances of the DHP(T1).

atoms are located at terminal position in the molecule and so are the most exposed to solvent. The presence of solvent increases the positive charge density at the H5 and H11 atoms. In contrast, the negative charge density at the oxygen atoms increases in the presence of solvent. The changes in the charge densities at these atomic sites are higher in polar solvents than in the nonpolar solvents. In dimer A the largest changes in the NAC values occur at the H and O atoms involved in the H-bonding interactions. The NAC of these H-sites are shifted to more positive charge density values while the two O-atoms are shifted to more negative values. Similar observations are found for dimer B where the O10, H11 and O24 atomic sites showed the maximum charge density variations compared to the monomer molecule (Table 6).

Fig. 5. The optimized molecular structure of the suggested isomers of DHP.

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Fig. 7. The optimized molecular structure of dimers A and B calculated using DFT/B3LYP method.

Molecular electrostatic potential (MEP) The molecular electrostatic potential (MEP) has been used primarily for predicting the reactive sites towards electrophilic and nucleophilic attack, and in studies of biological recognition and hydrogen bonding interactions [63,64]. The MEP of DHP(T1) calculated using B3LYP with a 6-311++G(d,p) basis set is shown in Fig. 8. The negative (red) regions of the MEP are related to electrophilic reactivity and the positive (blue) regions to nucleophilic reactivity. As can be seen from this figure, negative regions are mainly localized over the carbonyl oxygen (O12 = 0.0580 a.u) so it would be predicted that the carbonyl oxygen will be the most reactive sites for electrophilic attack, acting as the most H-acceptor site. The maximum positive region is localized on the hydrogen atoms of the OH (H11 = +0.0629 a.u) and NH (H5 = +0.0485 a.u) groups.

Hence, it is predicted that these H-sites will be the most reactive sites for nucleophilic attack and are the most H-donor sites [65]. Electronic absorption spectra and frontier molecular orbitals (FMOs) The first ten spin allowed singlet–singlet excitations were calculated using TD-DFT method in the gas phase and in solution in order to predict theoretically the possible electronic transitions of the most stable isomer (T1) and to show the effect of solvent polarity on its electronic spectra. The calculated kmax values with their major contributions of molecular orbitals to the electronic absorption bands for the T1 isomer are shown in Table 7. Only those electronic transitions which having the highest oscillator strength (f) values are considered. These spectral bands are calculated at 312.4, 218.8 and 215.7 nm in the gas phase.

Table 6 The calculated Natural charges of the DHP in the gas phase and in presence of solvents of different polarity using B3LYP/6-311++G(d,p) method.

*

Atom

Gas

CCl4

Cyclohexane

Acetonitrile

DMSO

Ethanol

Water

Dimer Aa

C1 H2 N3 N4 H5 C6 C7 C8 H9 O10 H11 O12

0.2078 0.2282 0.3119 0.3858 0.4022 0.4858 0.5857 0.2176 0.2298 0.6680 0.4841 0.6246

0.2117 0.2361 0.3200 0.3823 0.4197 0.4915 0.5879 0.2147 0.2391 0.6795 0.5027 0.6688

0.2112 0.2353 0.3192 0.3827 0.4178 0.4909 0.5878 0.2150 0.2381 0.6782 0.5006 0.6641

0.2191 0.2457 0.3283 0.3781 0.4451 0.4988 0.5884 0.2105 0.2514 0.6966 0.5318 0.7286

0.2193 0.2459 0.3283 0.3780 0.4457 0.4988 0.5883 0.2104 0.2515 0.6971 0.5325 0.7296

0.2199 0.2460 0.3282 0.3768 0.4465 0.4991 0.5883 0.2101 0.2518 0.6983 0.5337 0.7322

0.2187 0.2453 0.3280 0.3783 0.4441 0.4985 0.5885 0.2107 0.2508 0.6959 0.5305 0.7261

0.2093 0.2271 0.2973 0.3822 0.4516 0.4817 0.5920 0.2124 0.2288 0.6702 0.4847 0.6946

Bold values for the largest NAC variations. a One unit is considered due to symmetry consideration.

Dimer B Unit 1

Unit 2

0.2179 0.2236 0.3041 0.3852 0.3988 0.4907 0.5829 0.2142 0.2222 0.7126 0.5159 0.6397

0.2114 0.2404 0.3070 0.3669 0.4080 0.4916 0.5970 0.2052 0.2341 0.6615 0.4866 0.6661

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Fig. 9. The ground state isodensity surface plots for the frontier molecular orbitals contributed in the electronic transitions of the most stable isomer (T1).

Fig. 8. Molecular electrostatic potentials (MEP) mapped on the electron density surface calculated by the DFT/B3LYP method for the most stable isomer (T1).

Experimentally, the most intense transition band is observed in methanol at 310 nm [66]. Fig. 9 shows the isodensity surface plots of the molecular orbitals (MOs) contributing to this electronic transition. The most intense absorption band (312.4 nm) corresponds to the electronic transition from HOMO to LUMO with 81% contribution. Since the electron densities of these MOs are mainly localized on the p-system of the T1 molecule, this band belongs mainly to a p ? p* transition. In non-polar solvents, this spectral band showed little spectral shifts while in polar solvents it undergoes a hypsochromic shift. The other spectral absorption bands are almost unaffected by solvent. The HOMO–LUMO gap (DE) is an important parameter for predicting the nonlinear optical properties (NLO) of molecular systems. A low DE value indicates more ease of electronic transition and hence better NLO properties of a molecule. We make comparison with urea, a well known molecule used for comparative purposes to predict the NLO of molecular systems [67]. The DE value of the compound studied is calculated to be 4.4273 eV, which is lower than that of urea (6.7063 eV). From this point of view, the calculations predicted the studied compound have better NLO properties than urea. Unfortunately, the solid state structure showed that the crystal is centrosymmetric therefore is SHG inactive.

Natural bond orbital (NBO) analysis A natural bond orbital (NBO) analysis points to the most accurate natural Lewis structure of molecular system because all orbitals are mathematically included to obtain their highest possible percentage of the electron density. The natural bond orbital analysis of the significant NBO orbitals is given in Table S3 (Supplementary materials). In CAH bonds, the hydrogen atoms have almost 100% s-character. On the contrary, almost 100% pcharacter was observed in the lone pair of N4 and the second lone pair of O-atoms as well as on both atoms forming the double bonds. The NBO orbital analyses showed that all the CAN and CAO bond orbitals are polarized towards the nitrogen (0.768– 0.795 at N) and oxygen atom (0.797–0.841 at O) respectively. Therefore, the maximum electron densities on the oxygen and nitrogen atoms are responsible for the polarity of molecule. The NBO analysis results of dimers A and B are shown in Tables S4 and S5 (Supplementary materials) respectively. The NBO of BD(1)N4AH5 and BD(2)C7AO12 bonds in dimer A showed the most significant variations where the electron densities and the hybridization of both atoms are changed due to the H-bonding interactions. The percent electron density (%ED) at N4 is increased by about 3.89 while the %ED at H5 is decreased by the same magnitude. As a result the s-character at N4 is increased (2.71%) at the expense of the p-character (2.71%). Moreover, the %ED and the s-character at both atoms forming the BD(1)C7AO12 NBO are changed very little. In contrast, there were significant changes for the %ED at both atoms forming the BD(2)C7AO12. The (%ED) at C7 is decreased and at O12 is increased by about 3.34 compared to the monomer. We note that the formation of NAH  O H-bonding interactions in dimer A increases the electron

Table 7 The calculated electronic spectra of DHP(T1) using the TD-DFT method. Solvent

kmax

fosc

Major contribution

Solvent

kmax

fosc

Major contribution

Gas

312.4 218.8 215.7

0.0497 0.0025 0.0155

HOMO ? L (81%) H  3 ? L (31%), H  1 ? L + 2 (62%) H  2 ? L (40%), H ? L + 2 (51%)

DMSO

305.3 217.5 214.5

0.072 0.0313 0.0033

311.2

0.069

H ? L (83%)

304.7

0.0679

H ? L (83%) H  2 ? L (36%), H ? L + 1 (56%) H  3 ? L (40%), H  1 ? L + 1 (16%), H ? L + 2 (39%) H ? L (83%)

CCl4

220.7 216.9

0.0002 0.0295

H ? L + 1 (94%) H  2 ? L (36%), H ? L + 2 (55%)

Ethanol

217.2 214.5

0.0277 0.0035

311.5

0.0677

H ? L (83%)

304.1

0.0674

216.9 216.8

0.0037 0.0284

H  3 ? L (43%), H  1 ? L + 2 (48%) H  2 ? L (36%), H ? L + 2 (55%)

Water

217.3 214.5

0.0275 0.0028

304.4

0.0675

H ? L (83%)

217.2 214.4

0.0272 0.0033

H  2 ? L (37%), H ? L + 1 (55%) H  3 ? L (43%), H  1 ? L + 1 (17%), H ? L + 2 (35%)

Cyclohexane

Acetonitrile

H  2 ? L (37%), H ? L + 1 (55%) H  3 ? L (46%), H  1 ? L + 1 (19%), H ? L + 2 (30%) H ? L (83%) H  2 ? L (37%), H ? L + 1 (55%) H  3 ? L (34%), H  1 ? L + 1 (13%), H ? L + 2 (48%)

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Table 8 The second order perturbation energies E(2) (kcal/mol) of the most important charge transfer interactions (donor–acceptor) in the most stable isomer of the studied compound using B3LYP method. No. 1 2 3 4 5 6 7 8 9 10 a b c

Donor NBO (i) BD(2)C1AC8 BD(2)C1AC8 BD(2)N3vC6 LP(1)N3 LP(1)N3 LP(1)N4 LP(1)N4 LP(2)O10 LP(2)O12 LP(2)O12

Acceptor NBO (j) *

BD (2)N3AC6 BD*(2)C7AO12 BD*(2)C1AC8 BD*(1)N4AC7 BD*(1)C6AC8 BD*(2)N3AC6 BD*(2)C7AO12 BD*(2)N3AC6 BD*(1)C1AC7 BD*(1)N4AC7

E(2)a kcal/mol

E(j)–E(i)b a.u.

F(i,j)c a.u.

19.41 18.76 9.39 9.33 9.43 32.95 54.45 35.13 17.64 28.2

0.29 0.31 0.37 0.82 0.88 0.27 0.29 0.33 0.69 0.65

0.069 0.071 0.053 0.078 0.082 0.086 0.113 0.101 0.101 0.123

E(2) means energy of hyperconjucative interactions. Energy difference between donor and acceptor i and j NBO orbitals. F(i,j) is the Fock matrix element between i and j NBO orbitals.

density at the O-atoms while that at the H-atoms is decreased. The results are in good agreement with the NAC values obtained at these sites. Similar observations are predicted for the OAH  O H-bonding interactions in dimer B (Table S5, Supplementary materials). A NBO analysis has been performed on the molecule at the B3LYP/ 6-311++G(d,p) level in order to elucidate the intra- and intermolecular charge transfer interactions and delocalization of electron density within the molecules studied. The intra- and intermolecular charge transfer (ICT) interactions due to the delocalization of electron density between occupied NBO orbitals and the unoccupied ones lead to a stabilization of those interactions where the larger the E(2) value, the more intense is the interaction between electron

donors and electron acceptors, i.e. the greater donating tendency from electron donors to electron acceptors NBO and the greater the extent of conjugation of the whole system [68]. The energy of these interactions can be estimated by second-order perturbation theory [69]. In Table 8, the perturbation energies of the significant donor–acceptor interactions of the T1 isomer are presented. The LP(1)N4 ? BD*(2)C7AO12, LP(2)O10 ? BD*(2)N3AC6, LP(1)N4 ? BD*(2)N3AC6 and LP(2)O2 ? BD*(1)N4AC7 ICT interactions have the highest E(2) values of 54.45, 35.13, 32.95 and 28.2 kcal/mol respectively. In dimer A, the LP(1)N4 ? BD*(2)C7AO12 and LP(2)O12 ? BD*(1)N4AC7 ICT interaction energies are predicted to be 67.68 and 20.13 kcal/mol respectively. The former is stabilized while the latter

Table 9 The second order perturbation energies E(2) (kcal/mol) of the most important charge transfer interactions (donor–acceptor) in the dimers A and B using B3LYP method. Donor NBO (i)

Acceptor NBO (j)

E(2)a kcal/mol

BD*(2)N3AC6 BD*(2)C7AO12 BD*(2)C1AC8 BD*(1)N4AC7 BD*(1)C6AC8 BD*(2)N3AC6 BD⁄(2)C7AO12 BD*(2)N3AC6 BD*(1)C1AC7 BD⁄(1)N4AC7

19.54 20.71 10.03 9.35 9.60 34.71 67.68 34.63 17.44 20.13

BD(2)C13AC20 BD(2)C13AC20 BD(2)N15AC18 LP(1)N15 LP(1)N15 LP(1)N16 LP(1)N16 LP(2)O22 LP(2)O24 LP(2)O24

BD*(1)N16AH17 BD*(1)N16AH17

7.35 13.28

LP(1)O24 LP(2)O24

BD*(2)N3AC6 BD*(2)C7AO12 BD*(2)C1AC8 BD*(1)N4AC7 BD*(1)C6AC8 BD*(2)N3AC6 BD*(2)C7AO12 BD⁄(2)N3AC6 BD*(1)C1AC7 BD*(1)N4AC7

18.43 18.99 9.75 9.60 9.39 31.70 56.68 38.64 17.38 27.54

BD(2)C13AC20 BD(2)C13AC20 BD(2)N15AC18 LP(1)N15 LP(1)N15 LP(1)N16 LP(1)N16 LP(2)O22 LP(2)O24 LP(2)O24

BD*(1)O10AH11 BD*(1)O10AH11

6.19 13.74

No.

Donor NBO (i)

Acceptor NBO (j)

E(2)a kcal/mol

BD*(2)N15AC18 BD*(2)C19AO24 BD*(2)C13AC20 BD*(1)N16AC19 BD*(1)C18AC20 BD*(2)N15AC18 BD⁄(2)C19AO24 BD*(2)N15AC18 BD*(1)C13AC19 BD⁄(1)N16AC19

19.54 20.71 10.03 9.35 9.60 34.71 67.68 34.63 17.44 20.13

BD*(1)N4AH5 BD*(1)N4AH5

7.35 13.28

BD*(2)N15AC18 BD*(2)C19AO24 BD*(2)C13AC20 BD*(1)N16AC19 BD*(1)C18AC20 BD*(2)N15AC18 BD*(2)C19AO24 BD*(2)N15AC18 BD⁄(1)C13AC19 BD*(1)N16AC19

19.79 20.82 9.45 9.38 9.32 32.05 61.52 36.39 13.01 25.90

Dimer A Unit 1 BD(2)C1AC8 BD(2)C1AC8 BD(2)N3AC6 LP(1)N3 LP(1)N3 LP(1)N4 LP(1)N4 LP(2)O10 LP(2)O12 LP(2)O12

Unit 2

From unit 1 to unit 2 LP(1)O12 LP(2)O12

From unit 2 to unit 1

Dimer B Unit 1 BD(2)C1AC8 BD(2)C1AC8 BD(2)N3AC6 LP(1)N3 LP(1)N3 LP(1)N4 LP(1)N4 LP(2)O10 LP(2)O12 LP(2)O12

Unit 2

From unit 2 to unit 1 LP(1)O24 LP(2)O24 *

Bold for the most E(2) variations due to H-bonding interactions in dimers A and B.

No.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Gas

CCl4

Cyclohexane

DMSO

Acetonitrile

Water

Ethanol

t

A

t

A

t

A

t

A

t

A

t

A

t

A

3639 3462 3090 3068 1683 1600 1538 1417 1393 1304 1241 1194 1134 1082 967 958 822 796 779 708 652 613 514 480 463 436 361 353 304 114

89.4 69.6 0.2 1.3 412.6 286.8 8.3 65.6 30.2 7.8 36.9 138.6 71 58.7 0 56.1 42.1 33.4 9.5 0.1 75.6 7.3 0.4 4.1 21.8 141.6 1.6 8.5 4.1 3.1

3489 3347 3060 3041 1652 1584 1534 1415 1389 1304 1244 1197 1134 1082 967 960 824 800 782 711 654 613 514 483 465 426 365 354 304 119

193 150.7 1 0.1 400.6 609.1 9.6 88 57.6 11 19.8 189.6 111.9 75 0 84.4 53.7 41.5 20.8 0 94 10.1 3.9 7.5 31.6 167.6 8.6 7.6 7.7 3.9

3509 3362 3063 3043 1655 1586 1535 1415 1390 1304 1244 1197 1134 1083 967 960 824 800 782 711 654 613 514 482 465 426 365 354 304 119

178.8 139.8 0.7 0.1 409.7 563.8 9.4 85.3 53.8 10.4 21.2 184.9 106.7 73.8 0 81.2 52.4 40.7 19.3 0 91.7 9.8 3.7 7 30.5 164.1 7.7 7.7 7.5 3.8

3181 3135 3014 2995 1624 1547 1528 1409 1376 1299 1252 1195 1132 1081 965 961 823 803 785 716 652 613 517 484 468 403 369 353 305 123

490 348.4 11.4 4.4 170.1 1363.8 10.8 136 133.5 30.1 7.1 221.4 188.4 99.1 0 131.2 76.4 47.8 52.9 0 129.5 14.5 14.4 15.7 49.1 171.4 71.3 5.5 20.6 4.8

3193 3142 3013 2994 1625 1547 1528 1409 1375 1299 1252 1195 1130 1081 966 961 824 803 785 716 651 613 516 484 468 399 368 353 304 123

479.4 345.2 11 4.2 170.3 1353.3 12.4 136.6 130 31.2 7.2 218.1 182.9 102.5 0 130.6 76.2 48 51.9 0 128.5 14.5 15.3 15.6 48.9 158.1 80.4 5.7 22.5 4.7

3170 3127 3013 2994 1622 1543 1526 1408 1374 1298 1252 1195 1130 1081 965 961 823 804 785 717 653 613 516 485 467 392 361 351 301 123

503.9 353.2 11.6 4.6 160.4 1411.7 7.5 137.5 133 33.6 6.7 208.3 200.3 98.6 0 133.4 78.3 47.4 56.8 0 131.6 15 16.6 15.3 50.3 97.1 128.7 5.2 40.2 3.4

3207 3150 3016 2997 1626 1549 1528 1410 1376 1299 1252 1195 1130 1081 965 961 824 803 785 716 652 613 516 484 468 402 368 353 304 123

461.1 336 10.4 3.8 179.8 1321.6 10.9 133.8 126.1 29.2 7.5 218.7 180.7 99.9 0 128.5 74.9 48 50 0 127.2 14.3 14 15.1 48.1 168.3 68 5.8 20.5 4.8

Exp.

Assignment

3480

93tOH 81tNH 81tCH 79tCH 15tCC + 15tC@O + 13dHCC + 9dHNC 12tCC + 11tCN + 25dHCC 14tCN + 20tCC + 16dHCC 11tCN + 36dHCC 23dHNN + 23dHNC 9tCC + 36dHCC + 14dHCC 9tCAO + 26dHCC + 9dCCN 12tNN + 13tCC + 18tCN + 8tCAO + 7dHCC 23dHCC + 19dHOC 46dHCC 20sHCCN + 22sHCCO + 19sHCCC + 23sHCCH 13tCC + 12tCN + 19dHCC + 15dCCC + 14dCCN 27sHCCO + 25sHCCC + 16sHCCN 29tCC + 12tCN + 8tNN + 11dCCN 12tCC + 9tCN + 9tCO + 12dCNN + 15CCN + 11dNCO 16sHCCN + 17sHCCN + 24sCNNC 19sHNNC + 28sHNCC + 21sHNCO 11tCC + 18dCCC + 15CNN + 11dCCO 28sHNCC + 20sNCOH + 11sNNCC 20dCCO + 17dCCC + 10dNNC + 10dCCN + 12dNCO 21dCCO + 20dNCO + 18dHCC + 14dCCC + 15dNNC 23sNCOH + 25sCCOH 16sCNNC + 20sNNCC + 18sNNCO + 7sCCCC 21dNCO + 34dCCO 29sCCOO + 19sHNCC + 12sHCCO + 19sNNCO + 10sCCCC 15sCCCN + 16sHCCN + 11sHCCO + 15sCCCO + 23sCNNC + 16sNNCO

3047 2964 1664 1551 1406 1283 1136 1001 907

812 680 517

401 316

S.M. Soliman et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 1086–1098

Table 10 The calculated scaled vibrational frequencies and vibrational intensities of DHP(T1) in gas phase and in solvents of different polarities using B3LYP/6-311++G(d,p) method.

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is destabilized due to the H-bonding interactions between the NAH of one molecule as proton donor and the C@O of the other as proton acceptor. The formation of OAH  O H-bond in dimer B between an OH group of unit 1 and the C@O of unit 2 affect the LP(2)O10 ? BD*(2)N3AC6 and LP(2)O24 ? BD*(1)C13AC19 ICT interactions. The LP(2)O10 ? BD*(2)N3AC6 of unit 1 has E(2) value of 38.64 kcal/mol instead of 35.13 kcal/mol for the free molecule. On other hand the LP(2)O24 ? BD*(1)C13AC19 of unit 2 has E(2) value of 13.01 kcal/mol instead of 17.64 kcal/mol for the corresponding LP(2)O12 ? BD*(1)C1AC7 in the free molecule (Tables 8 and 9). It is clear for the present investigations that, the intermolecular H-bonding interactions stabilize the n ? p* ICT interactions while the n ? r* are destabilized. The intermolecular charge transfer interaction energies: (i) from monomer unit 1 to unit 2 (ii) from monomer unit 2 to unit 1 shown in Table 9 indicated that, the ICT interactions in dimer A from monomer unit 1 to unit 2 due to LP(1)O12/LP(2)O12 ? BD*(1)N16AH17 and from monomer unit 2 to unit 1 due to LP(1)O24/LP(2)O 24 ? BD*(1)N4AH5 stabilized the molecule by 13.28 kcal/mol. For dimer B, a stabilization of the molecule by 13.74 kcal/mol is due to LP(1)O24/LP(2)O24 ? BD*(1)O10AH11 interactions. These results confirm the presence of intermolecular NAH  O and OAH  O interactions in dimers A and B respectively.

Analysis of the vibrational spectra The calculated vibrational frequencies and intensities of the most stable isomer (T1) of the compound studied in the gas phase and in solutions of different polarities using the B3LYP/6311++G(d,p) method are given in Table S6 (Supplementary materials). The calculated vibrational wavenumbers are scaled by 0.9613 [70,71]. The assignment of these modes based on the total energy distribution (TED) is given in Table 10. The molecule consists of 12 atoms so it has 30 normal vibrational modes. The calculated scaled

values of these vibrational frequencies are now compared with the experimentally measured values. OAH and NAH vibrations. The OAH and NAH stretching vibrations of DHP(T1) are predicted at 3639 and 3462 cm1 respectively while experimentally observed at 3480 cm1. These stretching vibrations are predicted to be pure modes. The NAH in-plane and out-plane bending vibrational bands are calculated at 1393 cm1 and 652 cm1 respectively while OAH in-plane and out-plane bending vibrational bands are calculated at 1134 cm1 and 436 cm1 respectively. These scaled vibrational frequencies are in good agreement with the experimental data. CAH vibrations. The vibrational spectra showed the presence of CAH stretching vibrations in the range of 3047–2964 cm1. Two aromatic CAH stretching vibrations are predicted at 3090 and 3068 cm1. The TED contribution of these stretching modes indicates that these are rather pure modes, not mixed with other vibrations. The CAH in-plane bending vibrations are predicted in the range of 1683–958 cm1 (except modes 9 and 15) while the CAH out of plane torsion vibrations are predicted at lower frequency (modes 15, 17 and 20). C@O vibrations. The C@O stretching mode of the DHP(T1) is observed at 1664 cm1 in the experimental infrared spectrum. The t(C@O) mode was computed, using the B3LYP/6-311++G(d,p) method at 1683 cm1. The t(CAO) stretching vibration occur at a lower frequency than the t(C@O). The DFT calculation predicted the t(CAO) at about 1241–1194 cm1 while experimentally observed at 1283 cm1. CAC and CAN vibrations. The identification of the CAC and CAN vibrations is a difficult task, since the mixing of vibrations is

Table 11 The predicted effect of different solvents on the vibrational frequencies and intensities of DHP(T1). Mode

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Water

Ethanol

Acetonitrile

DMSO

CCl4

Cyclohexane

Dt

DA

Dt

DA

Dt

DA

Dt

DA

Dt

DA

Dt

DA

156 119 31 29 32 15 4 3 4 2 4 3 1 0 0 2 1 4 3 3 2 1 1 3 2 12 4 1 1 5

103.6 81.1 0.8 1.2 12.0 322.3 1.3 22.4 27.4 3.2 17.0 51.0 40.9 16.3 0.0 28.3 11.7 8.2 11.3 0.0 18.4 2.7 3.5 3.4 9.8 26.0 7.1 0.9 3.5 0.7

135 104 28 26 29 13 3 2 3 1 3 3 0 1 0 2 2 4 3 3 1 1 1 2 2 12 4 1 0 5

89.4 70.2 0.5 1.2 2.9 277.0 1.1 19.7 23.6 2.6 15.7 46.3 35.7 15.1 0.0 25.1 10.3 7.4 9.8 0.0 16.0 2.4 3.3 3.0 8.7 22.5 6.1 0.7 3.3 0.6

476 340 80 76 62 55 11 8 18 6 12 1 2 1 2 3 1 8 7 9 0 1 3 6 5 35 8 0 1 10

400.6 278.8 11.2 3.1 242.4 1077.1 2.5 70.4 103.4 22.3 29.7 82.8 117.4 40.3 0.0 75.2 34.3 14.4 43.3 0.0 53.9 7.1 14.0 11.7 27.3 29.8 69.7 2.9 16.5 1.7

462 331 81 77 61 54 11 8 19 6 12 1 4 1 1 3 2 8 6 9 1 1 2 6 4 39 7 0 0 10

390.0 275.6 10.8 2.9 242.2 1066.5 4.1 71.0 99.8 23.4 29.7 79.5 111.9 43.8 0.0 74.5 34.2 14.6 42.4 0.0 52.9 7.1 14.8 11.5 27.2 16.5 78.8 2.8 18.4 1.6

487 348 81 77 64 59 13 10 20 7 12 1 4 1 2 3 1 8 6 9 1 1 3 6 4 47 0 2 3 9

414.5 283.6 11.4 3.2 252.2 1124.9 0.8 71.9 102.8 25.8 30.2 69.8 129.3 39.9 0.0 77.3 36.2 14.0 47.3 0.0 55.9 7.6 16.2 11.2 28.5 44.5 127.1 3.3 36.1 0.2

448 323 77 73 61 52 11 8 17 6 11 1 3 1 2 3 1 7 6 8 0 1 3 5 4 37 8 0 0 10

371.7 266.4 10.1 2.5 232.8 1034.8 2.6 68.1 96.0 21.4 29.4 80.1 109.7 41.2 0.0 72.4 32.8 14.6 40.5 0.0 51.6 6.9 13.6 11.1 26.3 26.6 66.4 2.7 16.4 1.6

S.M. Soliman et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 1086–1098

possible in the relevant spectral region. The PED analysis showed that the CAC and CAN stretching, bending and torsion vibrations are mixed with other vibrations and with each other. Effect of solvent on the vibrational spectra. The solvent effects on the infrared vibrational frequencies and intensities are shown in Table 11 and Fig. S1 (Supplementary materials). The changes in the vibrational frequencies and intensities are small in nonpolar solvents. The OAH, NAH, CAH and C@O stretching modes showed the most significant variations in the vibrational frequencies and vibrational intensities. The predicted effect of solvent on the vibrational frequencies of the OAH and NAH stretching modes is great. These modes are shifted to lower frequencies and higher intensities compared to the gas phase. The low frequency shifts of the OAH and NAH stretching vibrations are higher in the presence of polar solvents such as water and ethanol than non-polar solvents (benzene and cyclohexane). The infrared intensities of these modes are found to increase up to a factor of 5 in the presence of polar solvents compared to the gas phase. Similar results have been predicted for the t(NAH) stretching vibrations. These results are in accordance with the stronger elongation of OAH and NAH bonds in the polar solvents relative to the nonpolar one (Table S2, Supplementary materials). The vibrational frequency shifts for the t(CAH) and t(C@O) stretching modes are small. Dimer forms. The analysis of the vibrational spectra of dimer A showed that the modes t(C@O) and t(NAH) involved in the intermolecular H-bonds change noticeably, especially the NAH stretching modes. For dimer A, the two t(NAH) modes (3101 and 3048 cm1) are calculated at lower frequencies compared to the free t(NAH) mode (3462 cm1). Similarly, the two H-bonded C@O groups undergo stretching vibrations at lower frequencies of 1661 and 1655 cm1 compared to the free C@O (1683 cm1). These reductions in stretching frequencies of the H-bonded NAH and C@O are in good agreement with the predicted elongation of these bonds due to H-bond formation (Table S2, Supplementary materials). Similarly, in dimer B, shifts to lower frequencies for the t(C@O) and t(OAH) due to the formation of OAH  O H-bonding interactions are predicted to be about 406 and 21 cm1 respectively. The t(C@O) and t(OAH) in dimer B are predicted to be 1663 and 3233 cm1 respectively instead of 1683 and 3639 cm1 in the monomer molecule. Conclusion The stabilities of the six possible DHP isomers in the gas phase and in solutions of different polarities are predicted using B3LYP/6311++G(d,p) calculations. In agreement with the measured X-ray crystal structure, the results showed that the most stable structure is the oxo-hydroxo form. Vibrational and electronic spectra in the gas phase and in solution were calculated at the same level of theory. The results showed that the NAH and OAH stretching modes are shifted to lower frequencies in presence of solvent. The calculated electronic spectra using TDDFT method showed that the most intense absorption band undergoes a hypsochromic shift in polar solvents compared to the gas phase. The molecular electrostatic potential map showed that the carbonyl oxygen is the most reactive site for electrophilic attack and acts as the most H-acceptor site while the hydrogen atoms of the OH and NH groups are the most reactive sites for nucleophilic attack and are the most Hdonor sites. Two dimer forms of DHP (A and B) were simulated where dimer A is stabilized by the stronger OAH  O H-bonding interactions than dimer B which has the weaker NAH  O interactions. The t(C@O), t(OAH) and t(NAH) bands undergo bathochromic shifts due to the H-bonding interactions. Natural bond orbital

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analysis was used to investigate the stabilization energy of various inter- and intramolecular interactions within the systems studied.

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