Does a hydrogen bonded complex with dual contacts show synergism? A matrix isolation infrared and ab-initio study of propargyl alcohol–water complex

Journal of Molecular Structure 1118 (2016) 147e156

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Does a hydrogen bonded complex with dual contacts show synergism? A matrix isolation infrared and ab-initio study of propargyl alcoholewater complex Jyoti Saini, K.S. Viswanathan* Department of Chemical Sciences, Indian Institute of Science Education and Research, Mohali 140306, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 March 2016 Received in revised form 3 April 2016 Accepted 4 April 2016 Available online 6 April 2016

When hydrogen bonded complexes are formed with more than one contact, the question arises if these multiple contacts operate synergistically. Propargyl alcohol-H2O complex presents a good case study to address this question, which is discussed in this work. Complexes of propargyl alcohol (PA) and H2O were studied experimentally using matrix isolation infrared spectroscopy, which was supported by quantum chemical computations performed at the M06-2X and MP2 level of theories, using 6-311þþG (d,p) and aug-cc-pVDZ basis sets. A 1:1 PA-H2O complex was identified in the experiments and corroborated by our computations, where the PA was in the gauche conformation. This complex, which was a global minimum, showed dual interactions, one of which was an n-s interaction between the OeH group of PA and the O of H2O, while the second was a H$$$p contact between the OeH group of H2O and the p system of PA. We explored if the two interactions in the 1:1 complex exhibited synergism. We finally argue that the two interactions showed antagonism rather than synergism. Our computations indicated three other local minima for the 1:1 complexes; though these local minima were not identified in our experiments. Atoms-in-molecules and energy decomposition analysis executed through LMO-EDA were also performed to understand the nature of intermolecular interactions in the PA-H2O complexes. We have also revisited the problem of conformations of PA, with a view to understanding the reasons for gauche conformational preferences in PA. © 2016 Published by Elsevier B.V.

Keywords: Propargyl alcohol Hydrogen bonding Conformations Matrix isolation Ab initio computations Infrared spectroscopy

1. Introduction Propargyl alcohol (PA) has been considered as a possible candidate molecule for detection in interstellar medium and has therefore been a subject of intense study [1]. It has also been popular as a probe for the study of both gas phase ion chemistry and surface chemistry on dust grains [1]. The isomer of PA, propenal, has been identified in the interstellar region [2], which therefore renders this molecule of astrochemical interest. Interestingly, the energy difference between the gaucheþ and gaucheconformers of PA in the submillimetre region allows for a sensitive detection of this molecule in various interstellar sources [1]. The molecule also has interesting implications in combustion chemistry, as two propargyl radicals combine to yield benzene, which eventually can provide a route for the production of

* Corresponding author. E-mail address: [email protected] (K.S. Viswanathan). http://dx.doi.org/10.1016/j.molstruc.2016.04.005 0022-2860/© 2016 Published by Elsevier B.V.

polycyclic aromatic hydrocarbons (PAHs) [3,4]. Tranter et al. using shock-tube experiments studied the kinetics of the reaction [5]. C3H3þC3H3/C6H6/PAH/soot The conformations of PA arise from the internal rotation of the CeOH group in PA, which results in two conformations: the gauche (g-PA) and the trans (t-PA) form [6]. Calculations at the CCSD(T)F12/VDZ-F12 level of theory [7] indicated the g-PA to be the lower energy conformer, with the higher energy trans structure (tPA) being about 6.7 kJ/mol above g-PA. Hirota proposed that the intramolecular hydrogen bond between the eOH group and p electrons, was the reason for the g-PA being the ground state conformer [8]. Subsequent atoms-in-molecules (AIMs) investigations did not support this explanation [9], as it was shown that there did not exist any bond critical point between the two functional groups mentioned above. There have been previous experimental microwave [6,10] and infrared (IR) studies on PA [11]. The microwave spectrum of PA has

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confirmed the occurrence of only the g-PA [6,7]. Similarly, for propargyl thiol (C3H4S) [12-15] and propargyl selenol (C3H4Se) [16], only the g-PA conformer has been observed. We have studied PA in an effort to understand its conformational preferences using matrix isolation infrared spectroscopy and ab initio computations. No study exists on this molecule using matrix isolation spectroscopy, a technique ideally suited for the study of conformations [17,18]. Ab initio computations were performed to obtain the structures of the conformers and their vibrational features. Natural bond orbital (NBO) analysis was also performed. Complexes of PA also pose interesting questions, given that the molecule presents a number of sites for weak non-covalent interactions. Arunan and group have reported the pure rotational spectra of Ar$$$PA complex [6], which was the first study on any weakly bound complex of PA. In this work, we have explored the 1:1 hydrogen bonded complexes of PA-H2O, using matrix isolation infrared spectroscopy and ab initio computations. Atoms-inmolecules (AIMs) and LMOEDA analysis were done to study the nature of bonding in complex. 2. Experimental details Matrix isolation experiments were performed using a Sumitomo closed cycle helium compressor-cooled cryostat (HC-4E1) to attain a temperature of 12 K. The experimental details have been described elsewhere [17-21]. Specifically in this experiment, N2 (Sigma Gases and Services 99%) and Ar (Sigma Gases and Services 99%) were used as matrix gases. PA (Sigma Aldrich 99%) and H2O (Millipore) was used without further purification and subjected to several freeze-pumpthaw-cycles before use. The sample-matrix gas mixture was prepared at the desired ratio using standard manometric procedures as described in the references mentioned above. Once the sample and matrix gas were deposited at 12 K, the spectra of the matrix isolated species were recorded using a Bruker Tensor 27 FTIR spectrometer, operating at a resolution of 0.5 cm1. After recording the spectrum, the matrix was warmed to ~30 K (for N2 matrix) and ~35 K (for Ar matrix), maintained at this temperature for about 30 minutes using a heater-temperature controller unit and recooled to ~12 K. The spectrum of the annealed matrix was again recorded. 3. Computational details The Gaussian-09 suite of programs [22] was used to compute the optimized geometries of PA. All computations were performed at M06-2X and MP2 level of theory, using 6-311þþG(d,p) and augcc-pVDZ basis sets. In the study of the PA-H2O complexes, the geometries of the monomers were first optimized. Using the optimized monomer geometries, the geometries of PA-H2O complexes were obtained. Vibrational frequency calculations were performed on the optimized structures of the monomers and the complexes, to confirm that the computed geometries did correspond to minima on the potential surface. The vibrational frequency calculations were used to assign the vibrational features observed in the experiments. The computed vibrational frequencies for different complexes were scaled on a mode-by-mode basis, by comparing the computed frequencies of the monomeric species, PA and water, with the observed experimental frequencies, in a given matrix, bringing them in agreement with experimental vibrational frequencies. The scaling factors thus derived from the spectra of the monomers, were then used in the analysis of the spectra of the complexes. The matrix perturb the modes in diverse spectral regions differently; and thus for these systematic differences, we

have used a varying mode-by-mode scaling [23,24]. The MP2 level frequencies matched very closely with our experimental frequencies, and hence we have used the frequencies computed at the MP2/aug-cc-pVDZ level to compare with our experiments. The energies of the complexes and the monomers were used to arrive at the interaction energies of the complexes. These interaction energies were corrected separately for BSSE (basis set superposition error), using Boys and Bernardi’s counterpoise method [25] and ZPE (Zero point energies). AIM2000 [26] was used to perform analysis on the charge density topology. This method proves to be very useful in identifying and characterizing hydrogen bonding interaction in PA-H2O complexes. We will first present our results on PA and its conformers, and then discuss our results on the PA-H2O hydrogen bonded complexes. 4. Results e PA conformers Even though the study of conformers of PA has been reported in the literature, we have revisited this problem in an effort to understand the reasons for the conformational preferences in PA. Experiments were performed using both Ar and N2 matrices, and the results in the two are comparable. We discuss only the results of our experiments using N2 matrix since the spectral features in the N2 matrix were sharper than that obtained in the Ar matrix. 4.1. Experimental Fig. 1 shows the infrared spectra of PA in N2 matrix, over the spectral region 3750-2850 cm1 corresponding to the OeH stretch, ≡CeH stretch and symmetric CH2 stretch, 1420-1370 cm1 corresponding to the (CH2 wag þ OeH bend), 1060-1020 cm1 corresponding to the CeO stretch in PA and 690-620 cm1 where the bending modes of PA occur, as shown in Table 1. As discussed in an earlier section, scaling factors mentioned as a footnote in Table 1 were calculated so as to bring to agreement, the experimental and computed wavenumber of vibrational modes of the gauche conformer of PA. For example, the factor 0.9548 for the region of the OeH stretch was obtained by dividing the experimental feature at 3641.9 cm1 of PA with the computed feature at 3814.4 cm1 corresponding to the OeH stretch in PA. Similarly, the CeO stretch and ≡CeH bend ⊥ to CeCeO plane were respectively used to obtain the scaling factors 0.9852 and 1.139 for the relevant regions of the spectra. Table 1 gives the optimized structures and assignment of the vibrational modes of conformers of PA observed in our experiments. Strong features of PA are observed at 3641.9, 3311.0, 2926.1/ 2881.2, 1040.8, 669.4 and 644.0 cm1. 4.2. Computational Computations performed at M06-2X and MP2 levels of theory, using 6-311þþG(d,p) and aug-cc-pVDZ basis sets indicated the gauche conformer of PA (g-PA) to be the ground state conformer. At all the above levels of theory, the gauche conformer optimized to two equivalent minima, corresponding to the CCOH dihedral angles of nearly ±50 . The trans conformer, with the CCOH dihedral angle of 180 , optimized to a minimum at all the levels of computation mentioned above, except at the MP2/6-311þþG(d,p) where this structure was indicated to be a saddle point. This saddle point connected two degenerate local minima with CCOH dihedral angles of ±163 , which are therefore referred to as near-trans structures, as indicated in Table 2. The energy barrier for the interconversion of these two equivalent near-trans MP2/6-311þþG(d,p) structures

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Fig. 1. Infrared spectra of PA over the region 3750-2850 cm1, 1420-1370 cm1, 1060-1020 cm1 and 690-620 cm1 a) Experimental spectra recorded at 12 K after deposition of PA in N2 matrix (sample: matrix ratio 3:1000) b) Spectra of (a) after annealing the matrix at 30 K.

Table 1 Experimental (N2 matrix at 12 K) and scaled computed wavenumbers (cm1) at MP2/aug-cc-pVDZ level for the gauche and trans conformers of PA and vibrational assignments. Experimental

Computed*

3641.9 3311.0 2926.1/2881.2 1389.8 1040.8 669.4 644.0

Assignment

Gauche (g-PA)

Trans (t-PA)

3642.0 3326.5 2926.5 1384.7 1040.8 683.3 644.1

3644.6 3329.6 2910.4 1411.1 1039.4 682.5 640.1

OeH stretch ≡CeH stretch CH2 symmetric stretch CH2 wag þ OeH bend CeO stretch ≡CeH bend in the CeCeO plane ≡CeH bend ⊥ to CeCeO plane

*Scaling factors: (3750-2850 cm1, 0.9548); (1420-1020 cm1, 0.9852); (690-620 cm1, 1.139).

Table 2 Energy Difference between PA conformers computed at different levels of theory. Level of theory

M06-2X MP2

Energy difference between g-PA and t-PA in kJ/ mol 6-311þþG (d,p)

aug-cc-pVDZ

8.70 8.16*

7.41 6.32

*This structure was near-trans. See text for details.

was 0.21 kJ/mol. However, at the MP2/aug-cc-pVDZ level, the trans conformer optimized to a minimum, with the CCOH dihedral angle of 180 . The relative energies of the conformers obtained at each of the above levels of computation are shown in Table 2. The energy difference between the global minimum gauche structure and the higher energy trans structure ranged from 6.32 to 8.70 kJ/mol. 5. Discussions e conformations of PA 5.1. Vibrational assignments Based on the computed energy difference between the two conformers and their respective statistical weights, the relative population for the two conformers was computed to be in the ratio 98:2 (gauche:trans). The vibrational features for PA observed in our experiments are therefore assigned to the dominantly populated gauche conformer. The strong absorptions of PA are at 3641.9 (OeH stretch), 3311.0 (≡CeH stretch), 2926.1/2881.2 (symmetric CH2

stretch), 1389.8 (CH2 wag þ OeH bend), 1040.8 (CeO stretch), 669.4 (≡CeH bend in the CeCeO plane) and 644.0 cm1 (≡CeH bend ⊥ to CeCeO plane), with the assignments for each feature given in parenthesis. These features and assignments are also shown in Table 1. The computed and experimentally observed vibrational frequencies for the gauche and trans conformer have similar values for the OeH stretch, ≡CeH stretch, CeO stretch, ≡CeH bend in the CeCeO plane and ≡CeH bend ⊥ to CeCeO plane; however, the bands at 2926.1/2881.2 and 1389.8 cm1 clearly point to the presence of the gauche conformer in the matrix. In the case of formic acid, higher energy conformer was found to interconvert to the lower energy form on exposure to the infrared radiation [27]. To examine if such a conformer interconversion could be induced between the gauche and trans structures of PA, we exposed the matrix to the infrared beam from the FTIR for 24 hrs at 12 K and 30 K to see if any interconversion to the trans structure could be observed, which would have been indicated by the appearance of the features near 2910.4 and 1411.1 cm1, which are unique to the trans conformer. However, no interconversion between the gauche conformer of PA to the trans structure could be observed in our experiments. 6. NBO analysis for the conformers of PA The role of the delocalization interactions was studied using NBO analysis to understand the conformational preferences in PA. NBO analysis displayed that in PA, both geminal and vicinal interactions are important in stabilizing both the gauche and trans conformations. The relative contributions of geminal and vicinal

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delocalization interactions in deciding the conformational preferences in PA were quantitatively estimated by systematically deleting the geminal and vicinal interactions, and computing the energy of the two conformers. Table 3 gives the energies of the two conformers of PA computed at the M06-2X/aug-cc-pVDZ level after deleting either or both of the geminal and vicinal interactions. The effect of deletion of both geminal and vicinal interactions is listed under the head ‘all’. The relative change in the energies as a result of systematic deletions of interactions is shown graphically in Fig. 2. It can be seen that when all delocalization interactions are operative, the gauche conformer is lower in energy than the trans conformer by 7.41 kJ/mol. When geminal interactions were deleted (and vicinal interactions retained), the energies of both conformers were raised, and the conformational ordering was the same as when all delocalization interactions were present, which therefore indicated that the geminal interactions were about equally operative in both conformers. However, when vicinal interactions were deleted (and geminal interactions retained), the trans conformer which was the higher energy conformer to begin with, now turned out to be lower in energy than the gauche; in other words there was an inversion in conformational preference. Deletion of the vicinal interaction increased the energy of the gauche conformer of PA by ~406 kJ/mol (~79%) and that of the trans conformer by ~393 kJ/mol (~77%), which indicated that vicinal interactions play an important role in deciding the conformational ordering in PA. When all the orbitals which are involved in delocalization interactions were deleted it was seen that the conformational ordering is the reverse of that obtained when all interaction were present; the inversion being dictated due to the absence of the vicinal interactions. It is therefore clear that vicinal orbital interactions play a key role in determining the conformational preferences in PA. A detailed list of vicinal and geminal interactions is given in Table S1 in supplementary information. Earlier studies had suggested the role of hydrogen bonding interactions as being operative in deciding the preference of PA to the gauche form. However, our AIM analysis on this molecule does not reveal any intra-molecular hydrogen bonding interaction, thereby ruling out the role of hydrogen bonding in the conformational landscape of PA; an observation reported earlier [9]. The H$$$p distance in the gauche-PA is calculated to be 2.95Å, which is rather large for an interaction of this type to be of any consequence. 7. Results e PAeH2O complex Experiments were performed using both Ar and N2 matrices; but as mentioned before we discuss only the results of our experiments using N2 matrix. 7.1. Experimental When PA and H2O were co-deposited in N2 matrix and the

Fig. 2. Correlation diagram showing the relative energies (in kJ/mol) of the gauche and trans conformer of PA, when orbital interactions were systematically deleted. Calculations were done at the M06-2X/aug-cc-pVDZ level. (The energy axis is not to scale.)

matrix then annealed at 30 K, new features were observed at 3703.4 cm1, 3467.2 cm1, 3306.7 cm1, 1604.0 cm1 and 1053.4 cm1. These features, marked with an asterix, can be seen in Fig. 3, which shows the IR spectra of the PA-H2O complex over the spectral region 3750-3250 cm1, 1650-1550 cm1 and 10601020 cm1. These product bands increased in intensity, when the concentration of either of the two reagents, PA or H2O, was increased which clearly indicates that these features are due to a complex involving PA and H2O. It must be mentioned that the above features assigned to the PA-H2O complex, were also observed, though with small intensities, even when no H2O was deliberately introduced in the matrix. Since H2O was a ubiquitous impurity in matrix isolation experiments, the features due to the H2O complex were observed even when only PA was deposited. However, the observation that the above features increased in intensity when the concentration of either of the two reagents was increased supports the assignment of these features to the PA-H2O complex. Furthermore, the observation of these features even at very low concentrations of PA and H2O implies that these features are likely due to a 1:1 complex of PA-H2O. 7.2. Computational Structures of the PAeH2O complex were computed at M06-2X and MP2 level of calculations using 6-311þþG(d,p) and aug-ccpVDZ basis sets. The results obtained at both the levels of theory were comparable and we present the structures of the complexes computed at the MP2/aug-cc-pVDZ level of theory in Fig. 4. The optimized structures of PA and H2O submolecules at same levels of theory were used as the starting structures for obtaining the complex geometries. For a comparison of our experimental data, we have considered only the complexes of H2O with gauche

Table 3 NBO analysis showing the energies obtained after deletion of the different delocalization interactions in conformers of PA, computed at M06-2X/aug-cc-pVDZ level. Conformer

Energy (Hartrees)*

Interactions deleted

Deletion energy (Hartrees)

Change in energy Hartrees

kJ/mol

g-PA

191.810948974

t-PA

191.808125401

Geminal Vicinal All Geminal Vicinal All

191.770222889 191.656637661 191.620661943 191.762838733 191.658291667 191.624115463

0.040726 0.154311 0.190287 0.045287 0.149834 0.184010

107.1 405.0 499.6 118.8 393.3 483.2

*Energies have not been corrected for ZPE.

Relative contribution for stabilization (%)

20.9 79.1 23.2 76.8

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Fig. 3. Infrared spectra of PAeH2O in a N2 matrix over the regions 3750-3250 cm1, 1650-1550 cm1, 1060-1020 cm1, of a) PA: H2O: N2 (0.8: 1.0:1000) at 12 K, b) PA: H2O: N2 (0.8: 1.0:1000) annealed at 30 K, c) PA: H2O: N2 (0.8: 0.0: 1000) annealed at 30 K, d) PA: H2O: N2 (0.0: 0.5: 1000) annealed at 30 K, e) PA: H2O: N2 (3.0: 0.4:1000) annealed at 30 K. The product bands have been marked with an asterix.

Fig. 4. Optimized geometries of the complexes of gauche-PA and water, computed at the MP2/aug-cc-pVDZ level (Dotted lines do not imply bond paths; these lines are drawn just to indicate the interaction in the complex).

conformer of PA, as the population of the trans conformer was too small to be observed. Complex 1 and complex 1*, are structures where the g-PA is a proton donor through hydroxyl hydrogen and a proton acceptor through its p cloud. H2O also serves as both a proton acceptor as well as proton donor. This structure has two hydrogen bonded interactions: one an OeH$$$O interaction (OeH of PA and oxygen of water) and a second OeH$$$p interaction, involving the H of H2O and the p system of PA, thus forming a cyclic complex. The complex 1 and 1* differ from each other only in the subtle orientation of the H2O molecule for H$$$p. While these structures do constitute two distinct isomeric and nearly isoenergetic forms with the energy difference of ~0.4 kJ/mol, it would not be experimentally possible to distinguish between these two minima. Hence for discussions on this structure of the PA-H2O complex, we will use complex 1. The

experimentally observed and scaled computed infrared features of the PA-H2O complexes are listed in Table 4. In complex 2, g-PA serves as the proton acceptor at its hydroxyl oxygen and H2O serves as the proton donor, with the H-bond interaction being of the OeH$$$O type. In complex 3, g-PA is the proton donor through ≡CeH and H2O serves as the proton acceptor, involving a CeH$$$O interaction. Another structure (complex 4) in which the p electron cloud of PA interacts with the H2O molecule, forming H$$$p complex, was also located. This structure differs from complex 1, in that it involves only the H$$$p interaction between PA and H2O and not the OeH$$$O interaction. Interaction energies for the complexes calculated at M06-2X and MP2 levels using 6-311þþG(d,p) and aug-cc-pVDZ basis sets are shown in Table 5. The zero-point energy corrected (EZPC) as well as BSSE corrected (EBSSE) interaction energies for these structures

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Table 4 Experimental and scaled computed vibrational wavenumbers (cm1) for PA, H2O, D2O, the different PA-H2O and PA-D2O complexes in N2 matrix at MP2/aug-cc-pVDZ level. Computeda

Experimental PA 3641.9 3311.0 1040.8 Water 3727.5 3635.0 1597.7 D2O 2765.9 2654.9 1178.5 a b

Complex 3467.2 (174.7)b 3306.7 (4.3) 1053.4 (12.6) Complex 3703.4 (24.1) e

PA 3642.0 3326.5 1040.8 Water 3760.3 3631.9

1604.0 (6.3) Complex 2748.9 (17.0) e

1597.6

1182.6 (4.1)

1178.5

D2O 2765.7 2629.4

Complex1 3508.8 (133.2) 3321.3 (5.2) 1054.0 (13.2) Complex1 3723.9 (36.4) 3575.1 (56.8) 1594.9 (2.7) Complex1 2737.0 (28.7) 2588.9 (40.5) 1175.0 (3.5)

Complex2 3631.0 (11.0) 3326.6 (0.1) 1022.3 (18.5) Complex2 3727.0 (33.3) 3519.7 (112.2) 1620.8 (23.2) Complex2 2734.9 (30.8) 2556.3 (73.1) 1189.2 (10.7)

Complex3 3642.1 (0.1) 3265.4 (61.1) 1037.4 (3.4) Complex3 3756.1 (4.2) 3629.8 (2.1) 1600.7 (3.1) Complex3 2762.8 (2.9) 2627.9 (1.5) 1181.3 (2.8)

Complex4 3641.3 (0.7) 3324.6 (1.9) 1036.6 (4.2) Complex4 3737.0 (23.3) 3600.2 (31.7) 1594.7 (2.9) Complex4 2747.9 (17.8) 2607.7 (21.7) 1175.2 (3.3)

Assignment OeH stretch in complex 1 ≡CeH stretch in complex 1 CeO stretch in complex 1

OeH asym. stretch in complex 1 OeH sym. stretch OeH bend in complex 1

O-D asym.stretch in complex 1 O-D sym. stretch O-D bend in complex 1

Scaling factors: (2780e2500 cm1, 0.9587); (1650e1550 cm1, 0.9850); (1200e1160 cm1, 0.9930). For other regions, scaling factors have already been given in Table 1. Dv ¼ v (complex)-v (monomer).

Table 5 Uncorrected/ZPE corrected/BSSE corrected interaction energies (in kJ/mol) for the PA-H2O complexes computed at the M06-2X and MP2 level using a 6-311þþG (d,p) and augcc-pVDZ basis set. M06-2X Complex 1 1* 2 3 4

MP2

6-311þþG (d,p) 38.1/28.9/34.3 38.1/28.9/34.3 27.2/19.2/25.1 16.3/9.6/13.4 18.4/12.6/16.7

aug-cc-pVDZ 34.3/25.1/32.6 33.5/24.7/31.8 24.3/15.9/22.2 13.8/6.7/11.7 17.6/11.3/15.5

are given in the same table. It can be seen that complex 1 is the most stable among all other structures at both M06-2X and MP2 levels of theory. Single-point calculations at the CCSD/aug-cc-pVDZ level were performed for the four complexes using the optimized geometries at the MP2/aug-cc-pVDZ. The uncorrected interaction energy of -29.7 kJ/mol computed at the CCSD level, for the complex 1 again corresponded to be the global minimum. The uncorrected interaction energies obtained were -23.4 kJ/mol for complex 2, followed by -15.1 kJ/mol for complex 4 and -14.6 kJ/mol for complex 3. Computed values of some important geometrical parameters at the MP2/aug-cc-pVDZ level for the different complexes indicated above are given in Table 6. 8. Discussions e PAeH2O complex For each of the gauchePA-water complexes, optimized at MP2

6-311þþG (d,p) 33.0/25.1/23.4 32.2/24.3/22.6 25.1/18.0/18.8 16.7/10.0/10.5 15.1/10.5/9.6

aug-cc-pVDZ 33.0/24.7/25.9 32.2/23.8/25.5 25.5/17.6/19.7 15.5/11.3/11.3 17.2/12.1/11.7

level of theory using aug-cc-pVDZ basis set, vibrational frequencies were computed and assigned to the different normal modes of vibration in the complexes, as indicated in Table 4. 8.1. Vibrational assignments 8.1.1. Features of PA submolecule in the PAeH2O complex 8.1.1.1. OeH stretch. Complex 1 is computed to show a red shift of 133.2 cm1, for the OeH stretch of PA relative to that in uncomplexed PA, while complex 2 and complex 4 were computed to show red shifts of 11.0 cm1 and 0.7 cm1 respectively, and complex 3 computed to show blue shift of 0.1 cm1 for the same mode. In our experiments, we observed a strong feature of the PA-H2O complex at 3467.2 cm1, which is red shifted by 174.7 cm1 from the OeH stretching feature of uncomplexed PA occurring at 3641.9 cm1. As can be seen from Table 4, the vibrational frequency shift in this mode agrees best with complex 1, strongly suggesting the

Table 6 Some important geometrical parameters for the different PA-H2O hydrogen bonded complexes computed at the MP2/aug-cc-pVDZ level, showing bond lengths (in Å), bond angles and dihedral angles (in o).

complex 1 complex 1* complex 4 complex 2

O9H8

O7H8O9

H10C1

H10C2

O9H10C1

O9H10C2

O7H8O9H10

1.95 1.95 O9H8 e O7H11 1.94

154.0 155.8 O7H8O9 e O7H11O9 153.0

2.45 2.43 H10C1 2.44 C4O7H11O9 10.8

2.56 2.53 H10C2 2.68

121.5 125.0 O9H10C1 120.4

149.9 152.0 O9H10C2 147.2 O9H3 2.17

28.5 4.4 C4C1H10O9 8.4 O9H3C2 178.5

complex 3

C2H3O9H10 27.1

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formation of this complex in the matrix. All other complexes are computed to have small shifts in this mode. 8.1.1.2. ≡CeH stretch. This mode in PA occurs experimentally at 3311.0 cm1 ; the same mode in the PA-H2O complex is indicated by our computations to be red shifted by 5.2 cm1 in complex 1, 61.1 cm1 in complex 3 and 1.9 cm1 in complex 4 and blue shifted by 0.1 in complex 2. Experimentally the product features are observed in the co-deposition experiments at 3306.7 cm1, red shifted by 4.3 cm1, which can be assigned to complex 1. 8.1.1.3. CeO stretch. In the co-deposition experiments, this mode is observed at 1053.4 cm1 in the complex, which is a blue shift of 12.6 cm1 from the same feature in uncomplexed PA. This shift is in excellent agreement with our computations for complex 1, which indicated this feature to be blue shifted by 13.2 cm1. It may be noted that the other complexes are computed to show a red shift for this mode, which therefore unequivocally supports our assignment for complex 1. 8.1.2. Features of H2O submolecule in the PAeH2O complex 8.1.2.1. H2O asymmetric stretch. The experimental feature for this mode in the complex is observed at 3703.4 cm1, amounting to a red shift of 24.1 cm1 from the same mode in uncomplexed H2O. This shift is consistent with the computed red shifts for complexes 1, 2 or 4. However, taken together with the conclusions made earlier, based on the other modes, we assign the 3703.4 experimental feature to complex 1. This assignment implies a computed shift of 36.4 cm1 for this complex as against an experimentally observed shift of 24.1 cm1. It must be recognized that this shift of ~36.4 cm1 for the computed asymmetric stretch of water, in complex 1, results from the dual role that water plays in this complex, as both a proton donor and acceptor. In a later section, we shall discuss if the two combined interactions imply cooperativity. 8.1.2.2. H2O bend. This mode of H2O in the PAeH2O complex is observed at 1604.0 cm1, which corresponds to a blue shift of 6.3 cm1 from the feature of uncomplexed H2O. However, this feature in complex 1 is computed to be red shifted by 2.7 cm1. 8.2. PA-D2O complexes In order to corroborate the assignments of the PA-H2O complex, we also performed experiments where PA was codeposited with D2O. The frequencies of the vibrational modes of the PA submolecule in the PA-H2O and PA-D2O complexes are not very different, as indicated both by our computations and our experiments. Hence we discuss only the modes of D2O in the PA-D2O complexes. 8.2.1. D2O asymmetric stretch In the PA-D2O complex, the D2O asymmetric stretch at 2748.9 cm1 is observed with a red shift of 17.0 cm1, from that in uncomplexed D2O (2765.9 cm1). While this shift does not unambiguously point to any one complex, the shift is consistent with our earlier conclusion that complex 1 is formed in the matrix. 8.2.2. D2O bend This mode is observed at 1182.6 cm1 in PA-D2O complex, which is blue-shifted from that in uncomplexed D2O (1178.5 cm1) by 4.1 cm1. However, as in OeH bend mode, complex 1 is computed to be red shifted by 3.5 cm1 from the O-D bend feature in uncomplexed D2O. It is clear that the experimentally observed features agreed best

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with the computed features for complex 1 (or 1*). In particular the OeH and CeO stretch of PA unambiguously point to the formation of complex 1 (or 1*) and hence it may be concluded that we were essentially trapping this complex in our matrix. 8.3. AIM analysis The charge density topology was examined using the atoms-inmolecules (AIM) theory of Bader [28]. (3, -1) bond critical points, (3, þ1) ring critical points that could be associated with different PA-H2O complexes were located, shown in Fig. 5. Useful quantities to characterize a bond are the electron density, rb, Laplacian (L) of electron density defined as the sum of the eigenvalues of the Hessian, S3i¼1 ¼ li, and ellipticity, ε, defined as l1/ l2 e 1, which provides a measure of the extent to which the charge is preferentially accumulated in given plane [29]. Table 7 lists these values computed at the bond critical points. The topological criteria of the existence of hydrogen bonding were proposed by Koch and Popelier. According to the criteria proposed by these authors, H-bonds should have an electron density at the H$$$Y BCP (rb) in the range 0.002e0.034 a.u. and the Laplacian of the electron density at H$$$Y BCP should be within the 0.024e0.139 a.u [30]. Moreover, the Laplacian at the BCP should be positive and charge density low, which is typical of closed-shell interactions such as ionic, hydrogen-bonding or van der Walls interaction. These interactions are dictated by the contraction of charge away from the interatomic surface towards each of the nuclei [28]. In all the PA-H2O complexes, the value of rb falls in the ranges proposed by Koch and Popelier. In complex 1, which shows both OeH$$$O and H$$$p interactions, the OeH$$$O hydrogen bond is relatively strong compared with the H$$$p interaction. For the H$$$p interaction, both the rb and Laplacian are close to the lower limit of the criteria proposed, which shows that they are very weak. NBO analysis was performed for the complex 1 at the MP2 level using aug-cc-pVDZ basis set to understand the role of the various orbital interactions in the stabilization of this complex. It was seen that there was a strong interaction between the lone pair of oxygen of H2O, which serves as the electron donor, and s* orbital of OeH of PA which serves as the electron acceptor, with the second order perturbation energy of 47.66 kJ/mol. The H$$$p interaction between the donor orbital of C≡C bond and the s* of the OeH of H2O was only 3.52 kJ/mol, implying that the former interaction was the dominant in this complex. 8.4. LMO-EDA of PA-H2O complexes To investigate the nature of hydrogen bonding interaction, an LMO-EDA [31] calculation with the MP2 method was carried out, and results are listed in Table 8. In LMO-EDA, total interaction energy DEMP2 is decomposed into five terms:

DEMP2¼DEesþDEexþDErepþDEpolþDEdisp where DEes is the electrostatic energy, DEex is the exchange energy, DErep is the repulsion energy, DEpol is the polarization energy and DEdisp is the dispersion energy. As shown in Table 8, the total interaction energy for MP2/augcc-pVDZ level (DEMP2) between PA and H2O is in the range of -33.97 to -15.61 kJ/mol. The DEMP2 (-33.97 kJ/mol) of complex 1 is the largest among all complexes. The largest stabilizing interaction in complex 1 at the MP2 level is the exchange interaction with energy of -66.44 kJ/mol. The second largest stabilizing interaction is electrostatic (-53.64 kJ/mol). Although the polarization energy (-17.36 kJ/mol) in complex 1 is the largest among all complexes,

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Fig. 5. Charge density topologies for the different structures of PA-water complexes computed at the MP2/aug-cc-pVDZ level.

Table 7 AIM calculations showing the charge densities rb and Laplacian V2rb at the bond critical point for various PA-water complexes at MP2/aug-cc-pVDZ level. The values of electron densities are expressed in a.u. Complex Complex 1 Complex 2 Complex 3 Complex 4

(A) OeH $$$p (B) OeH $$$O HeO$$$H ≡CeH$$$O OeH$$$p

rb

l1

l2

l3

V2rb

ε

0.0119 0.0250 0.0258 0.0146 0.0111

0.0103 0.0320 0.0333 0.0167 0.0097

0.0068 0.0299 0.0325 0.0156 0.0070

0.0529 0.1470 0.1559 0.0821 0.0523

0.0358 0.0851 0.0901 0.0498 0.0356

0.5147 0.0702 0.0246 0.0705 0.3857

Table 8 Energy decomposition analysis for the various PA-H2O complexes computed using the LMO-EDA method at the MP2/aug-cc-pVDZ level. All energies are in kJ/mol.

DEes

Complex Complex Complex Complex Complex

1 2 3 4

53.64 37.15 19.12 21.84

DEex (35%) (37%) (39%) (30%)

66.44 42.64 18.54 31.30

(43%) (41%) (38%) (43%)

DEpol makes a minor contribution to the total interaction energy between PA and H2O. Similarly, in other PA-H2O complexes, DEex and DEes makes major contributions to the total interaction energy (DEMP2) of complexes, while DEpol and DEdisp are the smallest components of the interaction energy at the MP2/aug-cc-pVDZ level. 9. Does a hydrogen bonded complex with dual contacts, such as that seen in PA-H2O, exhibit synergism; i.e. is there synergism between the OeH···O and OeH···p interaction? It has been pointed out that complex 1, which is the global minimum in the PA-H2O surface, shows two combined contacts; OeH$$$O and OeH$$$p. To probe if the two interactions behave synergistically, we compared the interaction energy of complex 1

Fig. 6. Optimised geometries of the complexes of transPA with water computed at the MP2/aug-cc-pVDZ level (Dotted lines do not imply bond paths; these lines are drawn just to indicate the interaction in the complex).

DErep

DEpol

DEdisp

DEMP2

118.70 76.69 33.22 54.10

17.36 (11%) 11.76 (11%) 6.48 (13%) 7.20 (11%)

15.23 (11%) 11.09 (11%) 4.73 (10%) 11.59 (16%)

33.97 25.94 15.61 17.78

with those of complexes 5 and 6, structures of which are shown in Fig. 6. The choice of complex 5 and 6 was prompted by the fact that complex 5 consisted of only the OeH/O interaction, while complex 6 only the OeH/p interaction; the same two interactions that are present together in complex 1. We used uncorrected energies at the MP2/aug-cc-pVDZ level for this comparison. (The same conclusions are reached if ZPE or BSSE corrected energies are used.) Complex 1 has interaction energy of -33.1 kJ/mol, while complexes 5 and 6 have interaction energies of -25.9 and -18.8 kJ/mol respectively. Complex 1 with two interactions operating simultaneously has a larger interaction energy than either complex 5 or 6. However, the sum of the interaction energies of complexes 5 and 6 is more than that for complex 1. Let alone synergism, the interactions do not even occur in an additive manner. The two interactions seem to operate antagonistically. Complex 1 has PA in the gauche conformation, while in complexes 5 and 6, PA exists in the trans conformation; yet the structures 1, 5 and 6, still serve as good choices for a comparison, as the conformation of PA is unlikely to make any substantial influence on the interaction energies of the complexes in question. See for example, complex 4 and 6, both of which have only a OeH$$$p interaction but with PA in gauche and trans conformations respectively. The two complexes have uncorrected interaction energies of -17.2 and -18.8 kJ/mol, respectively, at the MP2/aug-ccpVDZ level, which are clearly comparable. To understand the reason for the antagonism in complex 1, we examined the structures of the three complexes, 1, 5 and 6. The O9H8 bond in complex 5 is shorter (1.9075Å) than that in complex 1 (1.9511Å). Furthermore, the O7H8O9 angle is linear (179.7 ) in complex 5, whereas it is bent (154 ) in complex 1. It is known that

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hydrogen bonds prefer linear geometries, which therefore implies that complex 5 adopts the best geometrical parameters for hydrogen bonding; while in complex 1, both distances and angles have been compromised. The reason for this compromise in the distance and angle, seems to be due to a certain attempt by the two precursors to sacrifice optimum conditions for each of the two interactions, OeH$$$O and the OeH$$$p, in order to maximize the stabilization by allowing the two interactions to operate simultaneously. As a result, the interaction energy of complex 1 is less than the sum of interaction energies of complexes 5 and 6. Incidentally, the OeH$$$p distance though, is not very different in complex 1 and 6. The structural compromise therefore results in antagonism. To examine this point of antagonism further, we fixed the O9H8 bond length and O7H8O9 angle at the values obtained in complex 5 (i.e. 1.9075Å and 179.7 respectively), scanned the dihedral angle O7H8O9H10 and computed the energies for various structures. The best stabilization energy was found to be -26.4 kJ/mol, which is less than that obtained when a full geometry optimization was done to obtain complex 1. Incidentally, this energy is close to the interaction energy of complex 5, indicating that with the frozen parameters, the H$$$p interaction could not set in. These observations clearly reveal that the optimum parameters when single interactions were present, were not necessarily adopted in the complex when multiple interactions become operative. We also addressed the issue of antagonism by examining the shift in the asymmetric stretch of H2O in all the three complexes, all of which showed red shifts. While the frequency shift in complex 1 was 36.4 cm1, those in complexes 5 and 6 were 15.5 and 25.0 cm1. Again the combined shift in complex 5 and 6 of 40.5 cm1, was more than in complex 1. While it was not surprising that complex 1 with water involved in two interactions, manifested a greater frequency shift than either complex 5 or 6, one cannot help noticing the lack of additivity or synergism in complex 1.

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It was also recognized that where dual interactions were present, as in complex 1, the two precursors adopt a geometry that is not necessarily the one that is found when each of the interactions: OeH$$$O and OeH…p, were present individually. This results in antagonism, where the optimum geometries when individual interactions are present are in fact sacrificed when forming the dual interaction complex. It is worth mentioning that the global minimum computed for the PA-H2O system was structurally similar to the global minimum computed for the phenylacetylene-H2O system [20]. In the PA-H2O complex, we see a strong OeH$$$O interaction together with a relatively weaker OeH$$$p. In the phenylacetylene-H2O the global minimum was an OeH$$$p interaction together with a relatively weaker CeH$$$O interaction. In other words, both complexes show X-H$$$O (X ≡ C or O) together with an OeH$$$p interaction. However, while in the PA-H2O system, this global minimum was observed and the only one observed in the matrix, the computed global minimum was not observed in the matrix isolation experiments of phenylacetylene-H2O [20]. Rather in that experiment, an n-s complex, which was a local minimum involving the interaction between the acetylenic hydrogen of phenylacetylene and the oxygen of H2O was observed. Interestingly, the energy difference (BSSE corrected) between the global minimum and n-s in the phenylacetylene-H2O system was 2.99 kJ/mol only, while the energy difference between similar systems in PA-H2O was 14.64 kJ/mol. Clearly the two strong interactions in PA-H2O (i.e. OeH$$$O and OeH$$$p) direct the global minimum to be dominantly stronger than the corresponding global minimum in phenylacetylene-water. Interestingly acetylene, which lacks the secondary interaction, shows the n-s complex as the global minimum [32]. Clearly these systems highlight the importance of multiple interactions in the stabilization of complexes. Acknowledgements

10. Conclusions Infrared spectra of matrix isolated PA clearly indicate the presence of dominantly populated gauche conformer in the matrix. The experimental features are corroborated by computations. NBO analysis of PA conformers revealed that the vicinal interactions play a key role in the conformational preference. Furthermore, AIM analysis indicated that the intramolecular hydrogen bonding was not responsible for the gauche conformer to be the lower energy form in PA, as was thought earlier. We also studied the 1:1 complex of PA-H2O in the matrix. The FTIR spectra of the matrix isolated PAeH2O complex, together with ab initio computations, indicate that the global minimum (Complex 1) is a structure which involves two interactions; one between the OeH group of PA and O atom of water and another between the hydrogen of water and C≡C p system of PA. Convincing evidence for complex 1 was indicated by the large red shift of the OeH stretching vibration of the PA subunit. The cyclic geometry of the complex was confirmed by the AIM theory, which showed a ring critical point, consistent with the dual interactions mentioned above. NBO analysis on this complex indicated the existence of a strong interaction between the lone pair of oxygen of water, which serves as the electron donor, and s* orbital of OeH of PA which serves as the electron acceptor orbital. Three other local minima at MP2 level of theory were also located with an HeO$$$H interaction (in complex 2), a ≡CeH$$$O interaction (in complex 3) and an OeH$$$p interaction (in complex 4); however no experimental evidence was found in the matrix for these complexes. For all the complexes, electrostatic and exchange interactions are dominant contributors to the stabilization energy at the MP2/aug-cc-pVDZ level.

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