Conformational composition of propanol in gaseous state and in matrix isolation

Conformational analysis of the experimentally recorded IR absorption spectra of propanol in a gaseous state and in a low-temperature argon matrix at 20 K and at 35 K was carried out for different spectral ranges. It showed that the conformational composition of the samples in gas and in matrix isolation is different. In gaseous propanol, Gt conformers predominate, while in matrix isolation the most energetically favorable form is Tg conformer, which prevails in percentage at both considered temperatures of an Ar matrix.


Introduction
Propanol is one of the most useable alcohols for different applications, so it is widely studied both by experimental and theoretical methods. It is known that molecules of monohydric alcohols can form different cluster structures due to hydrogen bonding [1][2][3]. Moreover, each molecule of alcohol with more than one atom of carbon can be found in different conformations, which are formed as a result of rotation of atoms around chemical bonds [4][5][6]. In the case of propanol (structural formula CH 2 -CH 3 -CH 3 -OH), the molecule of which has two structural dihedrals, nine stable configurations exist: one plane structure and four pairs of enantiomers (or mirror-image pairs). Since enantiomers have similar energy and optical properties, the difference between them is usually neglected. As a rule, propanol conformers are designated by a symbolic title using a generally accepted scheme of dihedral angles CCCO+CCOH via big+small letters for trans-(T, t), gauche-(G, g) and gauche'-(G', g') conformers [6,7].
Conformational properties of propanol were studied in supersonic jets [6,8] and cryogenic matrices [6,9], in gas and liquid phases [7], in CCl 4 solutions at low concentrations [10]. The aim of the presented work is to analyze experimentally registered IR absorption spectra of propanol in gaseous state and in matrix isolation, and to compare conformational composition of the samples studied.

Experimental
Liquid propanol with purity > 99.9 from Fluka was used for experimental investigations. Atmospheric oxygen and nitrogen were removed from the alcohol by five freeze-pump-thaw cycles. Gas phase samples were obtained by the natural evaporation in vacuum process from the liquid surface.
FTIR spectra of propanol in gaseous state were measured using conventional single pass 10 cm gas cell equipped with KBr windows with the resolution 0.5 cm -1 using deuterated triglycine sulfate (DTGS) detector. The gas cell was filled with an alcohol using standard vacuum technique. Pressure of the alcohol in the cell was set close to its saturated pressure at 20 o C: 2 mbar.  [11][12][13]. In [14] the following assignment of the absorption bands to different propanol conformers was made: 3682 cm -1 -Gt conformer, 3679 cm -1 -Tt conformer, 3669 cm -1 -Gg', 3660 cm -1 -Tg and 3657 cm -1 -Gg. As is seen from Fig.1, one can distinguish an intense band at 3682 cm -1 , which can be assigned to vibrations of Gt conformer. In addition, there are bands at 3657 and 3669 cm -1 , which can be assigned to conformers Gg and Gg'. IR absorption bands of Tt and Tg conformers are not observed in this part of the spectrum, or their intensity is too low. Therefore, we can draw the conclusion that conformations with gauche-orientation of С -С bond -Gt, Gg and Gg'dominate in the studied sample of gaseous propanol.

Spectral region of OH stretching vibrations
Moreover, spectral bands of molecules, which are proton acceptors in dimers, with frequencies 3654, 3671 and 3675 cm -1 were found in [14]. All these three bands were found in our sample, too. In fig. 1, D denotes the dimer bands. Besides these identified bands there are other ones (for example, 3647 cm -1 ), which can belong to vibrations of free OH groups in bigger clusterstrimers and so on.
Spectral region 800 -1300 cm -1 In the spectral region from 800 to 1300 cm -1 the stretching vibrations of a carbon skeleton of the propanol molecule (i.e. С -С and С -О bonds) are located. Fig. 2 presents FTIR spectrum of gaseous propanol in the corresponding range. The stretching С -С vibrations of conformers with trans-configuration in respect to С -С bond (they are denoted as Тх) have frequencies 881 and 885 cm -1 [14], and the corresponding vibrations of conformers of Gx group are located in the range 850 -860 cm -1 . In the experimentally registered spectrum of gaseous propanol there are vibrations at these frequencies, but they are of low intensity.
In the spectrum presented in Fig. 2 the most intense absorption is observed at 1065 cm -1 . This band corresponds to a more high-frequency part of a doublet appearing at interaction of the stretching С -С and С -О vibrations [14]. However, this band gives few information about dominating conformations, since calculated frequencies for all five conformers are located in the range 1050 -1100 cm -1 . The second, lowfrequency component of the doublet, is more informative: vibrations of Тх conformers are observed at 1014 and 1025 cm -1 , while vibrations of Gx conformers are located between 965 and 972 cm -1 [14]. Therefore, the observed absorption maximum at 1020 cm -1 can be assigned to Тх vibrations, and the band at 972 cm -1to Gx vibrations.
The comparison of intensities of these two bands allows concluding that Gx conformers dominate in the investigated sample, which agree with the conclusions drawn from the analysis of the spectral region of the stretching ОН vibrations.
In addition to skeleton vibrations, in this region there are also bending OH vibrations. According to [14], bending OH vibrations in Gt conformer manifest at 1222 cm -1 , and the corresponding vibrations of Tt conformerat 1235 cm -1 . In the IR spectrum in Fig. 2 one can notice both of these bands, and an intensity of Gt band is higher than that of Tt band, indicating domination of Gt form over Tt one.  [15], the absorption band near 2890 cm -1 corresponds to the symmetric stretching С-Н vibrations in СН 3 group, and the band at 2972 cm -1to antisymmetric stretching С-Н vibrations in СН 3 group. Between them, the symmetric stretching С-Н vibrations of С β Н 2 group are located. According to the assignment from [16 -18], the band at 2942 cm -1 belongs to the conformer Tg, the band at 2952 cm -1to the conformer Gg. It means that the most intense vibrations in this region correspond to conformers, which have ОН group in gauche-position.

Spectral region of stretching С -Н vibrations
Absorption frequencies of conformers with hydroxyl group in trans-position are lower by several tens of wavenumbers. However, in the registered spectrum absorption at these frequencies is low. Tt and Gt conformers is essentially lower, indicating that the number of these conformers (with OH group in trans-position) in the sample is much less. It is interesting to note, that in the gas phase the domination of Gt conformer was observed in this spectral region. Apparently, during the cooling of the gas mixture and the deposition of the matrix, a transition from one form to another occurred, and the conformational composition of the sample changed. This is confirmed by the results of our quantum-chemical calculations [20], according to which the lowest energy barrier (about 3.2 kJ/mol) is observed between Gt and Gg 'conformers, that is, the transition from Gt conformation to Gg' conformation is the most probable.
In addition to the vibrations of propanol monomers, spectral bands of OH stretching vibrations of free (i.e., not involved in the formation of hydrogen bonds) hydroxyl groups of small propyl alcohol clusters are also observed in this frequency range. Such free hydroxyl groups are inherent in molecules that act as proton acceptors in the formation of larger dimers or open clusters. According to [14], IR absorption bands of propanol dimers appear at frequencies of 3654, 3671, and 3675 cm -1 . In the spectrum in Fig. 4 all three of these bands are present (denoted by the letter D), although they have a relatively low intensity. This fact indicates the presence of a small (compared to monomers) amount of dimers in the sample under study. Note that dimers were also present in the gas phase of propanol, as evidenced by the analysis of the spectra of gaseous propanol in the same spectral range (see Fig.1).
Registered maxima of IR bands of propanol monomers and dimers in an Ar matrix in the spectral region of the stretching vibrations of free hydroxyl groups as well as their assignments are listed in Table 1. Moreover, in the spectrum in Fig. 4 one can notice other low-intensity IR absorption bands, for example, at frequencies of 3643, 3644, 3648 and 3650 cm -1 , which most likely belong to clusters consisting of three or more molecules. Note that a band at 3647 cm -1 was also registered in gaseous propanol and was attributed to the vibrations of trimers. Thus, it can be concluded that both in gaseous propanol and during its isolation in a low-temperature argon matrix, not only individual propanol molecules in different conformations exist there, but also hydrogen-bonded clusters consisting of two, three and more molecules are formed.
It is interesting to consider how the conformational composition of the sample under study changes with an increase in the temperature of the argon matrix. In fig. 5 the IR absorption spectrum of propanol in an argon matrix at 35 K in the spectral region of stretching vibrations of the free hydroxyl group is presented. intense, similarly to the picture observed for the gas phase (see Fig. 3). Between these two vibrations, there are symmetric stretching vibrations of C β H 2 group. In contrast to the corresponding absorption bands in the gas phase, for which a strong overlap is observed (Fig. 3), the matrix isolation method allows the bands of different conformers to be separated.  In Fig. 6 one can distinguish five bands of С β Н 2 vibrations corresponding to five different propanol conformers [16,18]: the band of Tt conformer at 2907 cm -1 , the band of Gt conformer at 2918 cm -1 , the band of Gg' conformer at 2930 cm -1 , the one of Tg conformer at 2936 cm -1 and of Gg conformer at 2948 cm -1 . From the distribution of the relative intensities of these five bands, it can be concluded that conformers with gauche orientation of the hydroxyl group, i.e. conformers of the Xg/g' type, predominate in the sample under study. This conclusion completely coincides with the results of the analysis of the spectral region of OH vibrations for this sample, which additionally confirms the correctness of the conclusions made above.   After analyzing the ratio of the intensities of the IR absorption bands of various conformers in this part of the spectrum, it can be concluded that Gx-type conformers prevail in number over Tx-type conformers. In addition, there are slightly more Tt conformers than Tg conformers, and the number of Gg structures is slightly less than Gg '.

Spectral region of skeletal vibrations
When the sample was heated to 35 K, no significant changes in the spectral pattern were observed. Fig. 9 shows

Spectral region of bending ОН vibrations
Let us consider separately the spectral region 1100 -1300 cm -1 , where bending OH vibrations of propanol are manifested. The corresponding part of the recorded IR absorption spectra of propanol isolated in an argon matrix at temperatures of 20 K and 35 K is shown in Fig. 10 and 11, respectively. As can be seen, vibrations of all five propanol conformers are recorded in this region, the assignment of which was carried out on the basis of our quantum-chemical calculations, as well as the results of modeling presented in [14]. All the bands listed above are observed at both temperatures, but with slightly different intensity distributions. In addition, the spectrum contains bands at frequencies of 1240, 1246, and 1252 cm -1 . Since the intensity of these bands increases with increasing temperature, it can be assumed that they belong to propanol clusters.

Conclusions
Conformational analysis of the experimentally recorded IR absorption spectra of propanol in gaseous state and in a low-temperature argon matrix, carried out for different spectral ranges, showed that the conformational composition of the samples in these two cases is different.
Analyzing Thus, the results of our studies show that gaseous propanol contains the largest number of Gt conformers, which are the most stable of the five possible propanol conformers according to a number of quantum mechanical calculations [8,14,21]. The propanol molecules isolated in a low-temperature argon matrix are influenced by the environment; therefore, in this case the most energetically favorable form is Tg conformer, which prevails in percentage.