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The structural peculiarities of liquid n-heptanol and n-octanol
Journal of Molecular Liquids 169 (2012) 80–86
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
The structural peculiarities of liquid n-heptanol and n-octanol P. Golub ⁎, V. Pogorelov, I. Doroshenko Taras Shevchenko Kyiv National University, Faculty of Physics, 6, Academician Glushkov Prosp., Kyiv 03127, Ukraine
a r t i c l e
i n f o
Article history: Received 28 December 2011 Received in revised form 22 February 2012 Accepted 24 February 2012 Available online 8 March 2012 Keywords: Hydrogen bonding Alcohols Quantum-chemical calculations IR spectra
a b s t r a c t Different cluster structures of n-heptanol and n-octanol were analyzed by quantum-chemical methods. The optimized structures and IR spectra for clusters involving from 2 to 8 molecules were calculated. By estimation of the hydrogen bond strength the most profitable clusters in the sense of the number of molecules involving in the formation were established. Calculated IR spectra of both species were compared with experimental spectra registered for liquid phase under room temperature. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Nowadays alcohols become widespread substances for researching because they are very convenient for the studying of the processes of hydrogen bonding. Due to these processes complex structures, named clusters, comprising two, three and more molecules emerge in alcohols. These structures may be cyclic, when all hydroxyl groups take part in hydrogen bond formation, and chain-like, when one hydroxyl group on the edge of cluster remains non-bonded. Of course, especial attention was paid to methanol as the simplest representative of the alcohols. One of the most informative methods of the investigation of methanol is the matrix isolation (basically nitrogen, argon and neon matrices are used [1–5]). Another promising and powerful method is the researching of alcohol clusters in helium nanodroplets. This ingenious researching technique allows taking measurements for single clusters [6]. It was shown, that large cyclic clusters (larger than trimers) dominate in liquid alcohols. Analyzing the chemical shift of hydroxyl proton in methanol solved in carbon tetrachloride as a function of pressure at 200 °C and 400 °C and different methanol mole fractions [7] the preference of cyclic tetramers in the studied pressure and temperature range was showed. Using the method of infrared cavity ringdown spectroscopy with helium as a carrier gas the existence of all clusters larger than dimer in cyclic form was established [8]. The increasing of the large cluster fraction with the increasing of the methanol concentration was also shown. For more complicated alcohols the similar picture is observed. Molecular dynamics investigations were performed for neat tert-butanol ⁎ Corresponding author at: 56, Lomonosova St., Kyiv 03022, Ukraine. Tel.: + 380 30968352766. E-mail address: [email protected] (P. Golub). 0167-7322/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2012.02.016
and showed apparent microstructuring under ambient conditions [9]. Fourier transform infrared spectra of 2-propanol in jet expansions showed consistent decreasing of dimer fraction and significant increasing of larger cluster fraction with increasing of 2-propanol concentration from 0.1% to 0.5% [10]. However, some doubts about keeping this picture appear when coming to higher alcohols. With growing of the length of alkyl chains the steric repulsion also may grow. It is interesting to know when steric effects due to bulky alkyl chains become predominant over energetic expediency of large molecular aggregations forming. Simple spectra registration couldn't be fully satisfactory because the existence of the vibrational band assigned to the vibrations of free hydroxyl group may be due to the conversion of cyclic clusters into chain-like ones but not to their partitioning into dimers and monomers. In order to reach our goal we used the quantitative quantumchemical methods. The energy and IR spectra of heptanol and octanol for cyclic and chain-like clusters involving from two to eight molecules were simulated. Comparing the obtained results with the experimental spectra we tried to establish which type of clusters mainly exists in liquid phase in the case of higher alcohols. 1.1. Calculation and experimental details Quantum-chemical calculations were performed using the program package Gaussian03. Hartree–Fock method with 6–31G (d, p) basis set and DFT with B3LYP hybrid exchange correlation function and 6–31G (d, p) basis set were chosen as methods of ab initio calculations. The investigated substances were n-heptanol and n-octanol. Energy, equilibrium geometry and IR spectra of different cluster structures (cyclic and chain-like ones up to octomers) were calculated. For the choosing of the simulation method and the basis set a relative bulkiness of the simulated systems should be taken into account.
P. Golub et al. / Journal of Molecular Liquids 169 (2012) 80–86
Thus the complicated basis sets and the sophisticated methods lead to the undesirable growing of time spent on the calculations for such huge systems. In our case the well-known frequency scale factors 0.8929 [11] for HF/6–31G(d, p) and 0.9613 for B3LYP 6–31G(d, p) [12] give sufficiently good results with nice qualitative agreement with the experimental IR spectra. For the energy values we tried to establish only the general dependencies, such as which formations are energetically more favorable. The obtained results were compared with the results of similar calculations made for the same number of molecules but not involved in interaction. Dividing the difference on the number of hydrogen bonds in the cluster we obtain energy per one hydrogen bond. Of course, this is not pure energy of hydrogen bonds because electrostatic, van der Waals and other contributions are included in it. Nevertheless, this intrinsic interaction energy can be taken for the estimation of the hydrogen bond energy [13]. From the spectrum and from the energy it is clearly seen that hydrogen bonding really exists and the cluster is really formed. Experimental IR spectra were registered in the laboratory of Fourier-transform infrared spectroscopy in Vilnius University using the Bruker's FTIR spectrometer Vertex 70 with MCT detector. The spectra were registered in the spectral range 650–4000 cm − 1 with the resolution 4 cm − 1. 1.2. Results and discussion 1.2.1. N-heptanol There are a lot of possible conformers for each cluster structure. Determination of these structures is a hard task for which the using of comprehensive analysis by electronic-structure methods with high level of accuracy is needed, and it goes beyond the theme of our investigation. There is no conviction that the gotten geometries really correspond to the PES minimum and not to the saddle point. To minimize the error several cycles of optimization were performed for each type of clusters with different input geometries. Then the most appropriate results were chosen. In Fig. 1 the possible equilibrium structures for dimers, trimers and tetramers calculated by HF method are presented. IR spectra for n-heptanol chain-like and cyclic clusters, consisting of two, three, four and five molecules bonded consequently by
Fig. 1. Examples of possible cluster structures calculated for n-heptanol by HF 6-31G (d, p): a) chain-like formations, and b) cyclic formations. For better observing of the hydrogen-bonded part in the cluster long alkyl tails C7H15 are substituted by radical R.
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intermolecular hydrogen bonds were calculated by HF 6–31G(d, p) method and are presented in Fig. 2. The most interesting part of the spectra for us is the region of the hydroxyl group stretch vibrations (3000–4000 cm − 1), because the position of this band is the indicator of the existence and the strength of the hydrogen bonding between alcohol molecules. As it is seen from Fig. 2 for chain-like formations the number of the vibrational bands in the region of O\H stretch vibrations corresponds to the number of the molecules forming the cluster. The position of one band with the frequency 3745 cm − 1 does not depend on the number of molecules in the cluster. It corresponds to the vibrations of free OH group, not involved in the hydrogen bond formation. The other bands are shifted to the low frequency range and are assigned to the vibrations of the H-bonded hydroxyl groups. The value of their shift increases with the increasing of the number of molecules. In the case of dimer this band has the frequency of 3679 cm − 1, for three-molecular cluster they are 3631 and 3605 cm− 1, for tetramer 3631, 3618 and 3570 cm− 1, and for pentamer 3625, 3578, 3545, and 3530 cm− 1. For cyclic formations there are no free hydroxyl groups, so the vibrational band on frequency 3745 cm− 1 isn't observed. But the red shift of the H-bonded hydroxyl groups is still evident. For the cyclic trimer we have three bands with the frequencies 3645, 3638 and 3557 cm− 1, for cyclic tetramer — 3591, 3584, 3570 and 3517 cm− 1, and for pentamer — 3591, 3578, 3557, 3537 and 3517 cm− 1. It is clearly seen, that the average value of the red shift in the case of cyclic aggregations is larger than in the case of chain-like aggregations with the same number of molecules involved in the formation. The calculated by B3LYP method IR spectra of n-heptanol are presented in Fig. 3. Now the vibrational band of free hydroxyl group has the frequency 3660 cm − 1 with little deviations depending on the number of molecules. Its intensity is much weaker than in the case of HF calculations. The red shift of H-bonded vibrational bands is more pronounced. In the case of chain-like clusters such bands are observed at: for dimer — 3520 cm− 1, for trimer — 3337 and 3357 cm− 1, for tetramer — 3284, 3346 and 3377 cm− 1, and for pentamer — 3165, 3297, 3377 and 3389 cm− 1. In the case of cyclic clusters such bands are observed at: for trimer — 3346, 3408 and 3432 cm− 1, for tetramer — 3165, 3247, 3265 and 3320 cm− 1, and for pentamer — 3104, 3191, 3209, 3259 and 3290 cm− 1. Again the enlargement of the average value of the red shift in the case of cyclic aggregations in comparison with chainlike aggregations with the same number of molecules in cluster is observed. The results of electronic-structure calculations for O\H bond lengths and for average energies of hydrogen bonds for n-heptanol are presented in Tables 1 and 2, respectively. The lowest energy of hydrogen bonding corresponds to dimer. Then a huge “jump” is observed and after that the energy slowly increases and reaches the maximum for five–six molecules in the cluster. For lager aggregations the growing stops. In comparison with the previous results the average lengths of intramolecular O\H bonds increase and the average lengths of intermolecular O\H bonds decrease with the growing of the energies of hydrogen bonds. It means that with the increasing of their volume the heptanol clusters become more «tight» and, as a sequence, more stable. However, reaching the value of 4 or 6 molecules in the cluster the distances between O and H atoms remain practically the same both for the cyclic clusters and for the chain structures. So the most preferable structures should involve 4 or even 6 molecules with the equal probability for cyclic and chain-like formations. The larger aggregations shouldn't exist because steric repulsion is growing and no energetic benefit for hydrogen bonding is observed. Experimental spectrum of 1-heptanol in liquid phase compared with the predicted IR spectra by both methods for tetramers is presented in Fig. 4.
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P. Golub et al. / Journal of Molecular Liquids 169 (2012) 80–86
Fig. 2. IR spectra of n-heptanol calculated by HF for: a) chain-like clusters involving (from top to bottom) 2, 3, 4, 5 molecules; b) cyclic clusters involving (from top to bottom) 3, 4, 5 molecules.
Paying attention again at the region of O\H stretch vibrations (3000–4000 cm − 1) one can notice there a very broad band with the maximum near 3300 cm − 1, which belongs to the vibrations of the
H-bonded hydroxyls, and a very weak feature near 3700 cm − 1, corresponding to the vibration of free hydroxyl group. So large difference in the intensities of the bands at 3300 cm − 1 and 3700 cm − 1 can be
P. Golub et al. / Journal of Molecular Liquids 169 (2012) 80–86
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Fig. 3. IR spectra of n-heptanol calculated by B3LYP for: a) chain-like clusters involving (from top to bottom) 2, 3, 4, 5 molecules; b) cyclic clusters involving (from top to bottom) 3, 4, 5 molecules.
explained by the fact that in large clusters there is only one free hydroxyl group per 5 or 6 molecules involved in H-bond formation. In the calculated spectra for such clusters the absorption at 3603 cm − 1
is 20–30 times weaker than in the region 3200–3400 cm − 1 for HF method and for B3LYP this difference even enlarges. Moreover, in the cyclic clusters there are no free hydroxyls at all, and if we assume
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Table 1 The calculated lengths of intermolecular and intramolecular OH bonds of n-heptanol. Number of molecules
2 3 4 5 6 7 8
Cycle clusters
Chain-like clusters
HF/6-31G(d,p)
B3LYP/6-31G(d,p)
HF/6-31G(d,p)
B3LYP/6-31G(d,p)
Intramolecular OH bond
Intermolecular OH bond
Intramolecular OH bond
Intermolecular OH bond
Intramolecular OH bond
Intermolecular OH bond
Intramolecular OH bond
Intermolecular OH bond
– 0.950 0.953 0.954 0.954 0.953 0.953
– 2.015 1.905 1.882 1.880 1.887 1.891
– 0.982 0.989 0.991 0.991 0.989 0.988
– 1.854 1.736 1.701 1.694 1.710 1.711
0.945 0.950 0.951 0.952 0.951 0.952 0.952
2.034 1.957 1.938 1.912 1.913 1.910 1.903
0.974 0.980 0.985 0.986 0.984 0.986 0.983
1.885 1.816 1.768 1.760 1.762 1.742 1.750
Table 2 The calculated energies for n-heptanol hydrogen bonds. Number of molecules
2 3 4 5 6 7 8
Energy per one hydrogen bond in cycle clusters
Energy per one hydrogen bond in chain-like clusters
HF/6-31G(d,p), kcal/mol
B3LYP/6-31G(d,p) kcal/mol
HF/6-31G(d,p), kcal/mol
B3LYP/6-31G(d,p) kcal/mol
– − 6.07 − 7.37 − 7.78 − 7.95 − 7.89 − 7.67
– − 8.22 − 9.80 − 10.23 − 10.71 − 10.65 − 10.54
− 2.09 − 7.84 − 7.95 − 8.00 − 7.53 − 7.95 − 7.89
− 8.12 − 10.06 − 10.84 − 10.97 − 10.17 − 10.45 − 10.68
that open and closed structures exist in liquid phase in equal numbers (remembering the equal values of the bonding energies for the cycles and open clusters) then this difference becomes even twice larger. Comparing the calculated spectra with the experimental data one can notice that the position of vibrational bands assigned to the vibrations of H-bonded hydroxyls is much better described by B3LYP. Cyclic clusters show especially good coincidence with experiment. But at the same time B3LYP tend to overestimate the intensity of H-bonded bands while HF at least for chain-like clusters evaluate the relationship between intensities of all bands quite well. Also the position of CH3 vibrational bands is equal for experiment and HF calculations but
shifted on around 50 cm− 1 to the high frequency range in the case of B3LYP calculations. 1.2.2. N-octanol For the calculated spectra of n-octanol the same tendencies may be admitted as in the case of n-heptanol. For HF method the stable band with the frequency 3745 cm− 1 is observed for chain-like formations and is absent for cyclic formations. For B3LYP method the vibrational stretch band of free hydroxyl is shifted to the low frequency range and is located near 3660 cm− 1. For both methods the average value of red shift in the case of cyclic structures is larger than in the case of the
Fig. 4. Comparison of IR spectra of n-heptanol: a) from up to bottom the experimental spectrum, the calculated by HF one for chain-like tetramer, the calculated spectrum by HF for cyclic tetramer, and b) from up to bottom the experimental spectrum, the calculated by B3LYP one for chain-like tetramer, the calculated spectrum by B3LYP for cyclic tetramer.
P. Golub et al. / Journal of Molecular Liquids 169 (2012) 80–86
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Fig. 5. Comparison of different spectra of n-octanol: a) from up to bottom the experimental spectrum, the calculated by HF one for chain-like tetramer, the calculated spectrum by HF for cyclic tetramer, and b) from up to bottom the experimental spectrum, the calculated by B3LYP one for chain-like tetramer, the calculated spectrum by B3LYP for cyclic tetramer.
corresponding chain-like clusters. But for B3LYP the average red shift value is more drastic. In the case of pentamer these bands almost overlap with the vibrational bands assigned to the CH2 and CH3 groups of the alcohol molecule. The calculated energies and bond lengths are presented in Tables 3 and 4. The growing of the energy per one hydrogen bond with the increasing of the number of molecules in the cluster is still observed. And again this growing stops at the value of 4–5 molecules and then remains nearly stable. An obvious peak at the frequency 3746 cm− 1 is observed which is uniquely assigned to the stretching vibration of free hydroxyl groups (Fig. 5). It means that the relative number of chain-like structures and
monomers increases. The region of H-bonded hydroxyls again is described by B3LYP much better than by HF. But the position of free hydroxyl stretch band is shifted on the value a little less than 100 cm− 1, while the HF shows perfect coincidence. From the calculated energies it is seen that the relative fraction of chain-like trimers should grow giving the enlargement of the intensity of free hydroxyl stretch mode in the experimental spectrum. 1.3. Conclusions The quantum-chemical calculations of the structural and spectral characteristics of the cluster structure of n-heptanol were carried
Table 3 The calculated lengths of intermolecular and intramolecular OH bonds of n-octanol. Number of molecules
2 3 4 5 6 7 8
Cycle clusters
Chain-like clusters
HF/6–31G(d,p)
B3LYP/6–31G(d,p)
HF/6–31G(d,p)
B3LYP/6–31G(d,p)
Intramolecular OH bond
Intermolecular OH bond
Intramolecular OH bond
Intermolecular OH bond
Intramolecular OH bond
Intermolecular OH bond
Intramolecular OH bond
Intermolecular OH bond
– 0.950 0.952 0.953 0.954 0.953 0.953
– 2.019 1.921 1.884 1.879 1.887 1.891
– 0.982 0.987 0.991 0.991 0.990 0.989
– 1.877 1.855 1.749 1.704 1.691 1.703
0.947 0.950 0.951 0.952 0.952 0.952 0.952
2.001 1.961 1.938 1.903 1.916 1.910 1.903
0.974 0.980 0.983 0.986 0.985 0.986 0.985
1.877 1.816 1.791 1.744 1.756 1.742 1.747
Table 4 The calculated energies per one hydrogen bond for n-octanol clusters. Number of molecules
2 3 4 5 6 7 8
Energy per one hydrogen bond in cycle clusters
Energy per one hydrogen bond in chain-like clusters
HF/6–31G(d,p), kcal/mol
B3LYP/6–31G(d,p) kcal/mol
HF/6-31G(d,p), kcal/mol
B3LYP/6–31G(d,p) kcal/mol
– − 5.52 − 6.68 − 7.32 − 7.44 − 7.51 − 7.24
– − 8.26 − 9.70 − 10.59 − 10.67 − 10.80 − 10.54
− 5.21 − 6.96 − 6.92 − 7.48 − 7.29 − 7.22 − 7.33
− 7.60 − 10.45 − 10.31 − 10.73 − 10.75 − 10.60 − 10.68
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out using the possibilities of the program set Gaussian 03. The optimal geometries, bonding energies and IR spectra for different heptanol and octanol clusters were calculated. Taking into account the results of electronic structure calculation (the frequencies of the vibrational bands corresponding to the bonded hydroxyl groups in the clusters of different sizes; the more energy profitability of larger clusters and the equal bonding energies for cycles and open structures) we conclude that in liquid phase in n-heptanol there are in equal proportions cyclic and open clusters consisting of more than 4 molecules. For n-octanol the increasing of the contribution of the chain-like structures is observed. It looks like beginning from octanol the steric repulsion begins to play a significant role and competes on equal terms with the hydrogen bonding. This conclusion is confirmed by the difference in the intensities of the absorption bands corresponding to the vibrations of free and bonded OH-groups in the experimental IR spectrum of liquid heptanol and octanol. Both used methods for ab initio calculations have their own advantages and disadvantages. The using of B3LYP is much better for simulating the region of H-bonded hydroxyls while stretch vibration of free hydroxyl groups and CH3 groups is described much precisely by HF. B3LYP tend to overestimate the intensities of H-bonded vibrational bands in several times while the overestimation by HF is not so large especially for chain-like aggregations. Acknowledgment The work was supported by Ukrainian State Found of Fundamental Researches in the frame of the international project F41/138-2011.
References [1] S. Coussan, A. Loutellier, J.P. Perchard, S. Racine, A. Peremans, A. Tadjeddine, W.Q. Zheng, The Journal of Chemical Physics 107 (1997) 17. [2] Z. Mielke, S. Coussan, K. Mierzwicki, P. Roubin, M. Saldyka, The Journal of Chemical Physics 110 (2006) 4712–4718. [3] J.P. Perchard, Chemical Physics 332 (2007). [4] V. Pogorelov, I. Doroshenko, P. Uvdal, V. Balevicius, V. Sablinskas, Molecular Physics 108 (17) (2010) 2165–2170. [5] I. Doroshenko, V. Pogorelov, V. Sablinskas, V. Balevicius, Journal of Molecular Liquids V157 (2010) 142–145. [6] J. Peter Toennies, Andrey F. Vilesov, Angewandte Chemie International Edition 43 (2004) 2622–2648. [7] Claus Czeslik, Jiri Jonas, Chemical Physics Letters 302 (1999) 633–638. [8] R.A. Provencal, J.B. Paul, K. Roth, C. Chapo, R.N. Casaes, R.J. Saykally, G.S. Tschumper, H.F. Schaefer, The Journal of Chemical Physics 110 (1999) 9. [9] Larisa Zoranic, Franjo Sokolic, Aurelien Perera, The Journal of Chemical Physics 127 (2007) 024502. [10] Holger Schaal, Thomas Haber, Martin A. Shum, The Journal of Physical Chemistry. A 104 (2000) 265–274. [11] J.A. Pople, A.P. Scott, M.W. Wong, L. Radom, Israel Journal of Chemistry 33 (1993) 345. [12] M.W. Wong, Chemical Physics Letters 256 (1996) 391. [13] Katharina Wendler, Jens Thar, Stefan Zahn, Barbara Kirchner, The Journal of Physical Chemistry. A 114 (2010) 35.
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
The structural peculiarities of liquid n-heptanol and n-octanol P. Golub ⁎, V. Pogorelov, I. Doroshenko Taras Shevchenko Kyiv National University, Faculty of Physics, 6, Academician Glushkov Prosp., Kyiv 03127, Ukraine
a r t i c l e
i n f o
Article history: Received 28 December 2011 Received in revised form 22 February 2012 Accepted 24 February 2012 Available online 8 March 2012 Keywords: Hydrogen bonding Alcohols Quantum-chemical calculations IR spectra
a b s t r a c t Different cluster structures of n-heptanol and n-octanol were analyzed by quantum-chemical methods. The optimized structures and IR spectra for clusters involving from 2 to 8 molecules were calculated. By estimation of the hydrogen bond strength the most profitable clusters in the sense of the number of molecules involving in the formation were established. Calculated IR spectra of both species were compared with experimental spectra registered for liquid phase under room temperature. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Nowadays alcohols become widespread substances for researching because they are very convenient for the studying of the processes of hydrogen bonding. Due to these processes complex structures, named clusters, comprising two, three and more molecules emerge in alcohols. These structures may be cyclic, when all hydroxyl groups take part in hydrogen bond formation, and chain-like, when one hydroxyl group on the edge of cluster remains non-bonded. Of course, especial attention was paid to methanol as the simplest representative of the alcohols. One of the most informative methods of the investigation of methanol is the matrix isolation (basically nitrogen, argon and neon matrices are used [1–5]). Another promising and powerful method is the researching of alcohol clusters in helium nanodroplets. This ingenious researching technique allows taking measurements for single clusters [6]. It was shown, that large cyclic clusters (larger than trimers) dominate in liquid alcohols. Analyzing the chemical shift of hydroxyl proton in methanol solved in carbon tetrachloride as a function of pressure at 200 °C and 400 °C and different methanol mole fractions [7] the preference of cyclic tetramers in the studied pressure and temperature range was showed. Using the method of infrared cavity ringdown spectroscopy with helium as a carrier gas the existence of all clusters larger than dimer in cyclic form was established [8]. The increasing of the large cluster fraction with the increasing of the methanol concentration was also shown. For more complicated alcohols the similar picture is observed. Molecular dynamics investigations were performed for neat tert-butanol ⁎ Corresponding author at: 56, Lomonosova St., Kyiv 03022, Ukraine. Tel.: + 380 30968352766. E-mail address: [email protected] (P. Golub). 0167-7322/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2012.02.016
and showed apparent microstructuring under ambient conditions [9]. Fourier transform infrared spectra of 2-propanol in jet expansions showed consistent decreasing of dimer fraction and significant increasing of larger cluster fraction with increasing of 2-propanol concentration from 0.1% to 0.5% [10]. However, some doubts about keeping this picture appear when coming to higher alcohols. With growing of the length of alkyl chains the steric repulsion also may grow. It is interesting to know when steric effects due to bulky alkyl chains become predominant over energetic expediency of large molecular aggregations forming. Simple spectra registration couldn't be fully satisfactory because the existence of the vibrational band assigned to the vibrations of free hydroxyl group may be due to the conversion of cyclic clusters into chain-like ones but not to their partitioning into dimers and monomers. In order to reach our goal we used the quantitative quantumchemical methods. The energy and IR spectra of heptanol and octanol for cyclic and chain-like clusters involving from two to eight molecules were simulated. Comparing the obtained results with the experimental spectra we tried to establish which type of clusters mainly exists in liquid phase in the case of higher alcohols. 1.1. Calculation and experimental details Quantum-chemical calculations were performed using the program package Gaussian03. Hartree–Fock method with 6–31G (d, p) basis set and DFT with B3LYP hybrid exchange correlation function and 6–31G (d, p) basis set were chosen as methods of ab initio calculations. The investigated substances were n-heptanol and n-octanol. Energy, equilibrium geometry and IR spectra of different cluster structures (cyclic and chain-like ones up to octomers) were calculated. For the choosing of the simulation method and the basis set a relative bulkiness of the simulated systems should be taken into account.
P. Golub et al. / Journal of Molecular Liquids 169 (2012) 80–86
Thus the complicated basis sets and the sophisticated methods lead to the undesirable growing of time spent on the calculations for such huge systems. In our case the well-known frequency scale factors 0.8929 [11] for HF/6–31G(d, p) and 0.9613 for B3LYP 6–31G(d, p) [12] give sufficiently good results with nice qualitative agreement with the experimental IR spectra. For the energy values we tried to establish only the general dependencies, such as which formations are energetically more favorable. The obtained results were compared with the results of similar calculations made for the same number of molecules but not involved in interaction. Dividing the difference on the number of hydrogen bonds in the cluster we obtain energy per one hydrogen bond. Of course, this is not pure energy of hydrogen bonds because electrostatic, van der Waals and other contributions are included in it. Nevertheless, this intrinsic interaction energy can be taken for the estimation of the hydrogen bond energy [13]. From the spectrum and from the energy it is clearly seen that hydrogen bonding really exists and the cluster is really formed. Experimental IR spectra were registered in the laboratory of Fourier-transform infrared spectroscopy in Vilnius University using the Bruker's FTIR spectrometer Vertex 70 with MCT detector. The spectra were registered in the spectral range 650–4000 cm − 1 with the resolution 4 cm − 1. 1.2. Results and discussion 1.2.1. N-heptanol There are a lot of possible conformers for each cluster structure. Determination of these structures is a hard task for which the using of comprehensive analysis by electronic-structure methods with high level of accuracy is needed, and it goes beyond the theme of our investigation. There is no conviction that the gotten geometries really correspond to the PES minimum and not to the saddle point. To minimize the error several cycles of optimization were performed for each type of clusters with different input geometries. Then the most appropriate results were chosen. In Fig. 1 the possible equilibrium structures for dimers, trimers and tetramers calculated by HF method are presented. IR spectra for n-heptanol chain-like and cyclic clusters, consisting of two, three, four and five molecules bonded consequently by
Fig. 1. Examples of possible cluster structures calculated for n-heptanol by HF 6-31G (d, p): a) chain-like formations, and b) cyclic formations. For better observing of the hydrogen-bonded part in the cluster long alkyl tails C7H15 are substituted by radical R.
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intermolecular hydrogen bonds were calculated by HF 6–31G(d, p) method and are presented in Fig. 2. The most interesting part of the spectra for us is the region of the hydroxyl group stretch vibrations (3000–4000 cm − 1), because the position of this band is the indicator of the existence and the strength of the hydrogen bonding between alcohol molecules. As it is seen from Fig. 2 for chain-like formations the number of the vibrational bands in the region of O\H stretch vibrations corresponds to the number of the molecules forming the cluster. The position of one band with the frequency 3745 cm − 1 does not depend on the number of molecules in the cluster. It corresponds to the vibrations of free OH group, not involved in the hydrogen bond formation. The other bands are shifted to the low frequency range and are assigned to the vibrations of the H-bonded hydroxyl groups. The value of their shift increases with the increasing of the number of molecules. In the case of dimer this band has the frequency of 3679 cm − 1, for three-molecular cluster they are 3631 and 3605 cm− 1, for tetramer 3631, 3618 and 3570 cm− 1, and for pentamer 3625, 3578, 3545, and 3530 cm− 1. For cyclic formations there are no free hydroxyl groups, so the vibrational band on frequency 3745 cm− 1 isn't observed. But the red shift of the H-bonded hydroxyl groups is still evident. For the cyclic trimer we have three bands with the frequencies 3645, 3638 and 3557 cm− 1, for cyclic tetramer — 3591, 3584, 3570 and 3517 cm− 1, and for pentamer — 3591, 3578, 3557, 3537 and 3517 cm− 1. It is clearly seen, that the average value of the red shift in the case of cyclic aggregations is larger than in the case of chain-like aggregations with the same number of molecules involved in the formation. The calculated by B3LYP method IR spectra of n-heptanol are presented in Fig. 3. Now the vibrational band of free hydroxyl group has the frequency 3660 cm − 1 with little deviations depending on the number of molecules. Its intensity is much weaker than in the case of HF calculations. The red shift of H-bonded vibrational bands is more pronounced. In the case of chain-like clusters such bands are observed at: for dimer — 3520 cm− 1, for trimer — 3337 and 3357 cm− 1, for tetramer — 3284, 3346 and 3377 cm− 1, and for pentamer — 3165, 3297, 3377 and 3389 cm− 1. In the case of cyclic clusters such bands are observed at: for trimer — 3346, 3408 and 3432 cm− 1, for tetramer — 3165, 3247, 3265 and 3320 cm− 1, and for pentamer — 3104, 3191, 3209, 3259 and 3290 cm− 1. Again the enlargement of the average value of the red shift in the case of cyclic aggregations in comparison with chainlike aggregations with the same number of molecules in cluster is observed. The results of electronic-structure calculations for O\H bond lengths and for average energies of hydrogen bonds for n-heptanol are presented in Tables 1 and 2, respectively. The lowest energy of hydrogen bonding corresponds to dimer. Then a huge “jump” is observed and after that the energy slowly increases and reaches the maximum for five–six molecules in the cluster. For lager aggregations the growing stops. In comparison with the previous results the average lengths of intramolecular O\H bonds increase and the average lengths of intermolecular O\H bonds decrease with the growing of the energies of hydrogen bonds. It means that with the increasing of their volume the heptanol clusters become more «tight» and, as a sequence, more stable. However, reaching the value of 4 or 6 molecules in the cluster the distances between O and H atoms remain practically the same both for the cyclic clusters and for the chain structures. So the most preferable structures should involve 4 or even 6 molecules with the equal probability for cyclic and chain-like formations. The larger aggregations shouldn't exist because steric repulsion is growing and no energetic benefit for hydrogen bonding is observed. Experimental spectrum of 1-heptanol in liquid phase compared with the predicted IR spectra by both methods for tetramers is presented in Fig. 4.
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Fig. 2. IR spectra of n-heptanol calculated by HF for: a) chain-like clusters involving (from top to bottom) 2, 3, 4, 5 molecules; b) cyclic clusters involving (from top to bottom) 3, 4, 5 molecules.
Paying attention again at the region of O\H stretch vibrations (3000–4000 cm − 1) one can notice there a very broad band with the maximum near 3300 cm − 1, which belongs to the vibrations of the
H-bonded hydroxyls, and a very weak feature near 3700 cm − 1, corresponding to the vibration of free hydroxyl group. So large difference in the intensities of the bands at 3300 cm − 1 and 3700 cm − 1 can be
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Fig. 3. IR spectra of n-heptanol calculated by B3LYP for: a) chain-like clusters involving (from top to bottom) 2, 3, 4, 5 molecules; b) cyclic clusters involving (from top to bottom) 3, 4, 5 molecules.
explained by the fact that in large clusters there is only one free hydroxyl group per 5 or 6 molecules involved in H-bond formation. In the calculated spectra for such clusters the absorption at 3603 cm − 1
is 20–30 times weaker than in the region 3200–3400 cm − 1 for HF method and for B3LYP this difference even enlarges. Moreover, in the cyclic clusters there are no free hydroxyls at all, and if we assume
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Table 1 The calculated lengths of intermolecular and intramolecular OH bonds of n-heptanol. Number of molecules
2 3 4 5 6 7 8
Cycle clusters
Chain-like clusters
HF/6-31G(d,p)
B3LYP/6-31G(d,p)
HF/6-31G(d,p)
B3LYP/6-31G(d,p)
Intramolecular OH bond
Intermolecular OH bond
Intramolecular OH bond
Intermolecular OH bond
Intramolecular OH bond
Intermolecular OH bond
Intramolecular OH bond
Intermolecular OH bond
– 0.950 0.953 0.954 0.954 0.953 0.953
– 2.015 1.905 1.882 1.880 1.887 1.891
– 0.982 0.989 0.991 0.991 0.989 0.988
– 1.854 1.736 1.701 1.694 1.710 1.711
0.945 0.950 0.951 0.952 0.951 0.952 0.952
2.034 1.957 1.938 1.912 1.913 1.910 1.903
0.974 0.980 0.985 0.986 0.984 0.986 0.983
1.885 1.816 1.768 1.760 1.762 1.742 1.750
Table 2 The calculated energies for n-heptanol hydrogen bonds. Number of molecules
2 3 4 5 6 7 8
Energy per one hydrogen bond in cycle clusters
Energy per one hydrogen bond in chain-like clusters
HF/6-31G(d,p), kcal/mol
B3LYP/6-31G(d,p) kcal/mol
HF/6-31G(d,p), kcal/mol
B3LYP/6-31G(d,p) kcal/mol
– − 6.07 − 7.37 − 7.78 − 7.95 − 7.89 − 7.67
– − 8.22 − 9.80 − 10.23 − 10.71 − 10.65 − 10.54
− 2.09 − 7.84 − 7.95 − 8.00 − 7.53 − 7.95 − 7.89
− 8.12 − 10.06 − 10.84 − 10.97 − 10.17 − 10.45 − 10.68
that open and closed structures exist in liquid phase in equal numbers (remembering the equal values of the bonding energies for the cycles and open clusters) then this difference becomes even twice larger. Comparing the calculated spectra with the experimental data one can notice that the position of vibrational bands assigned to the vibrations of H-bonded hydroxyls is much better described by B3LYP. Cyclic clusters show especially good coincidence with experiment. But at the same time B3LYP tend to overestimate the intensity of H-bonded bands while HF at least for chain-like clusters evaluate the relationship between intensities of all bands quite well. Also the position of CH3 vibrational bands is equal for experiment and HF calculations but
shifted on around 50 cm− 1 to the high frequency range in the case of B3LYP calculations. 1.2.2. N-octanol For the calculated spectra of n-octanol the same tendencies may be admitted as in the case of n-heptanol. For HF method the stable band with the frequency 3745 cm− 1 is observed for chain-like formations and is absent for cyclic formations. For B3LYP method the vibrational stretch band of free hydroxyl is shifted to the low frequency range and is located near 3660 cm− 1. For both methods the average value of red shift in the case of cyclic structures is larger than in the case of the
Fig. 4. Comparison of IR spectra of n-heptanol: a) from up to bottom the experimental spectrum, the calculated by HF one for chain-like tetramer, the calculated spectrum by HF for cyclic tetramer, and b) from up to bottom the experimental spectrum, the calculated by B3LYP one for chain-like tetramer, the calculated spectrum by B3LYP for cyclic tetramer.
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Fig. 5. Comparison of different spectra of n-octanol: a) from up to bottom the experimental spectrum, the calculated by HF one for chain-like tetramer, the calculated spectrum by HF for cyclic tetramer, and b) from up to bottom the experimental spectrum, the calculated by B3LYP one for chain-like tetramer, the calculated spectrum by B3LYP for cyclic tetramer.
corresponding chain-like clusters. But for B3LYP the average red shift value is more drastic. In the case of pentamer these bands almost overlap with the vibrational bands assigned to the CH2 and CH3 groups of the alcohol molecule. The calculated energies and bond lengths are presented in Tables 3 and 4. The growing of the energy per one hydrogen bond with the increasing of the number of molecules in the cluster is still observed. And again this growing stops at the value of 4–5 molecules and then remains nearly stable. An obvious peak at the frequency 3746 cm− 1 is observed which is uniquely assigned to the stretching vibration of free hydroxyl groups (Fig. 5). It means that the relative number of chain-like structures and
monomers increases. The region of H-bonded hydroxyls again is described by B3LYP much better than by HF. But the position of free hydroxyl stretch band is shifted on the value a little less than 100 cm− 1, while the HF shows perfect coincidence. From the calculated energies it is seen that the relative fraction of chain-like trimers should grow giving the enlargement of the intensity of free hydroxyl stretch mode in the experimental spectrum. 1.3. Conclusions The quantum-chemical calculations of the structural and spectral characteristics of the cluster structure of n-heptanol were carried
Table 3 The calculated lengths of intermolecular and intramolecular OH bonds of n-octanol. Number of molecules
2 3 4 5 6 7 8
Cycle clusters
Chain-like clusters
HF/6–31G(d,p)
B3LYP/6–31G(d,p)
HF/6–31G(d,p)
B3LYP/6–31G(d,p)
Intramolecular OH bond
Intermolecular OH bond
Intramolecular OH bond
Intermolecular OH bond
Intramolecular OH bond
Intermolecular OH bond
Intramolecular OH bond
Intermolecular OH bond
– 0.950 0.952 0.953 0.954 0.953 0.953
– 2.019 1.921 1.884 1.879 1.887 1.891
– 0.982 0.987 0.991 0.991 0.990 0.989
– 1.877 1.855 1.749 1.704 1.691 1.703
0.947 0.950 0.951 0.952 0.952 0.952 0.952
2.001 1.961 1.938 1.903 1.916 1.910 1.903
0.974 0.980 0.983 0.986 0.985 0.986 0.985
1.877 1.816 1.791 1.744 1.756 1.742 1.747
Table 4 The calculated energies per one hydrogen bond for n-octanol clusters. Number of molecules
2 3 4 5 6 7 8
Energy per one hydrogen bond in cycle clusters
Energy per one hydrogen bond in chain-like clusters
HF/6–31G(d,p), kcal/mol
B3LYP/6–31G(d,p) kcal/mol
HF/6-31G(d,p), kcal/mol
B3LYP/6–31G(d,p) kcal/mol
– − 5.52 − 6.68 − 7.32 − 7.44 − 7.51 − 7.24
– − 8.26 − 9.70 − 10.59 − 10.67 − 10.80 − 10.54
− 5.21 − 6.96 − 6.92 − 7.48 − 7.29 − 7.22 − 7.33
− 7.60 − 10.45 − 10.31 − 10.73 − 10.75 − 10.60 − 10.68
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out using the possibilities of the program set Gaussian 03. The optimal geometries, bonding energies and IR spectra for different heptanol and octanol clusters were calculated. Taking into account the results of electronic structure calculation (the frequencies of the vibrational bands corresponding to the bonded hydroxyl groups in the clusters of different sizes; the more energy profitability of larger clusters and the equal bonding energies for cycles and open structures) we conclude that in liquid phase in n-heptanol there are in equal proportions cyclic and open clusters consisting of more than 4 molecules. For n-octanol the increasing of the contribution of the chain-like structures is observed. It looks like beginning from octanol the steric repulsion begins to play a significant role and competes on equal terms with the hydrogen bonding. This conclusion is confirmed by the difference in the intensities of the absorption bands corresponding to the vibrations of free and bonded OH-groups in the experimental IR spectrum of liquid heptanol and octanol. Both used methods for ab initio calculations have their own advantages and disadvantages. The using of B3LYP is much better for simulating the region of H-bonded hydroxyls while stretch vibration of free hydroxyl groups and CH3 groups is described much precisely by HF. B3LYP tend to overestimate the intensities of H-bonded vibrational bands in several times while the overestimation by HF is not so large especially for chain-like aggregations. Acknowledgment The work was supported by Ukrainian State Found of Fundamental Researches in the frame of the international project F41/138-2011.
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