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Benzyl alcohol—an unusual vibrational interaction
Spectrochimica Acta. 1961, Vol. 17, pp. 12.0 to 133. Pergamon Press Ltd.
Benzyl alcohol-an
Printed in Northern Ireland
unusual vibrational interaction
J. C. EVANS Chemical Physics Research Laboratory The Dow Chemical Company, Midland, Michigan (Received 7 October1960) Abstract-Another example of an unusual interaction [I, 21 has been studied in detail in the vibrational spectra of benzyl alcohol. Infrared and Raman spectra of related molecules, including deuterated derivatives, were recorded to assist in the interpretation.
Introduction examples have been reported [ 1, 21 of unusual regions of distortion which occur in broad, infrared absorption bands of materials in condensed phases. It was proposed that these features are the result of interactions between fundamental vibrational modes and that such effects may be expected to occur whenever a relatively sharp energy level falls within the energy range of a much broader level. Comparisons of observed and calculated band envelopes were made [2]. In all of the examples previously presented the broad vibrational level involved was that of the NH, wagging mode in several aromatic amines; this mode yields a band of half-width about 150 cm-l in the 500-800 cm-l region and it is readily modified by change of molecular environment or by deuterium substitution. The other, sharp level involved was variable and in no case was a definite assignment to a particular vibrational mode possible. An example in which the interacting modes are an aromatic CH bending mode and a mode of the -CH,Ogroup has been found in benzyl alcohol. The results of a study of this case are presented here. SEVERAL
Experimental Commercial benzyl alcohol (b.p. 203-205°C) was used. C,D,.CH,OH was prepared by exchanging sodium benzoate with D,O in a sealed tube at 13O”C, in the presence of pre-reduced platinum oxide, for 24 hr [3], followed by conversion to benzoic acid and reduction of this to the alcohol using lithium aluminum hydride [a]. The exchange step was repeated twice before proceeding to the acid. The final product was estimated from its infrared spectrum to be approximately 85 per cent deuterium substituted, which was suffidiently pure for the present purpose. C,H,.CH,OD was made by direct exchange of benzyl alcohol with D,O. Infrared spectra were recorded using a prism-grating instrument designed and constructed in this laboratory [Fi]; spectral slit widths used were about 0.5 cm-l. [I] J. C. EVANS and N. WRIQHT, S~ecl~oochi~.Acta 16, 352 (1960). [2] [3] [4] [5]
J. C. EVANS, Spectrochim. Acta 16, 1382 (1960). W. G. BROWN and J. L. GAFCNETT, J. Am. Chem. SW 80, 5272 (1958). R.F. NYSTROM andW.G. BROWN, J. Am. Chem Sot. 69,254s (1947). L. W. HERSCHER,S~~C~~OC~~WS. Acta 15, 901 (1959).
1
129
J. C. EVANS
Raman spectra were recorded using a Hilger photoelectric instrument and 4358 A excitation (7 A/mm). Some spectra were also recorded photographically on the same instrument. Wavenumber values quoted in the text are believed to be better than &-2 cm-l.
Results and discussion The spectral data are illustrated in Figs. 1,2 and 3,in which the examples of the effect are pointed out by arrows. The most striking feature is the extremely sharp turning point at the peak of the narrow transmission region within the broad
Fig. l(a). The infrared spectrum (full line) and the Raman spectrum (dotted line) of liquid C,H,.CH,OH. The Raman spectrum is drawn on an arbitrery, linear, relative-intensity scele, while the optical-density scale refers to the infrared spectrum.
CM” Ce H;CH,OH
C,H,CH,OH _I
\
v1:‘; I
i I I “IL’
CS,
I 1200
I
I
_I
a 0
-
I’ 1
co.,:
-
Vapor
solution
I.0 -
,O l i, 1000
v1;
V
II
-
I’
I
1200
800
-CM-
Fig. l(b). Tbeinfraredspeotrumof CIH,.CH,OH in CS,; (i) 10 per cent solution in 0.106mm cell; (ii) 0.5 per cent solution in l-14-mm cell.
130
I
I
I I
I I
IO00
I
I 17.
800
c M-’
Fig. l(c). TheinfrctredspectrumofC,H,.CH,OH vapor in a lo-cm cell at 126°C with vapor pressure*; (i) that of the liquid at -lOO°C and (ii) at - 120°C.
Benzy
alcohol-an
unusual vibration&l i&era&ion
absorption band; this establishes that the effect is not merely the result of the overat non-in~r~t~~g bands. Theo~ti~l con~deratio~ [Z] indicated lap of two adj such sharp turning points, but the total effect observed in this case deviates somewhat from the predictions of simple theory. Whereas the effect observed here is reminiscent of the special case in whioh the sharp level coincides with the center of the broad level it is apparent from the figures that this condition is not satisfied. (0)
C,D;CH,OH II
11
11
II
(b)
II
O-
CM-’
Fig. 2. The infrared spectrum of a thin of liquid C*D~.~~*OH between KBr pl
Fig. 3. (a) The infrared speotrum of 8 liquid film of @-methy benzyl alcohol at ~70°C. (b) The ;aframd speutrkn of p-methyl benzyl alcohol in CS,; 5 per cent solution in O*l-mm toll.
However, the situation is probably complica~d by the finite intensity of the transition betweea the ground level and the sharp level in those molecules in which the interaction is weak. Fig. 1(a) shows that the effect in benzyl alcohol coincides with a strong Raman band at 1030 cm-l. All monosubstituted benzene derivatives show such a Raman band with the co~espondin~ sharp infrared band [ti] which are assigned to an inplane CH bending mode
The broad, strong band in the infrared spectrum of benzyl alcohol (half-width approx. 60 am-l) is very weak in the Raman spectrum. It undoub~dIy arises from a mode of the 4X,0group since it ap ars in the infrared spectra of 1R] R. R. RANDLE and D. H. WHIFFEN.
of Petroleum,
London (1955).
Inslituta ofPdmkumCo&rence,
x31
Lodon,
1954. p. 111. Institute
J.C.EVANS C,D,.CH,OH and of C,H,.CH,OD; in the infrared spectrum of the related molecule, C,H,.CH,SH, the broad band appears near 975 cm-l while the 1030 cm-l band is isolated and easily observed. Of the four CH, bending modes only one is to be expected in this general spectral region, the twisting mode. The high intensity of the broad band indicates however that the mode responsible is not a simple one and probably involves the entire -CH,O--group. A detailed knowledge of the nature of the mode is not essential for our purposes. The other band in this region-the very strong Raman band at 1004 cm-l is the phenyl ring breathing mode. It is invariably very weak in the infrared and, since it does not appear to interact with the CH,O mode it will not be discussed further. When the ring hydrogen atoms are replaced by deuterium the modes involving ring hydrogen motion are moved considerably to lower wavenumber and the effect should disappear. In accord with this, the effect is absent from the infrared spectrum of C,D,.CH,OH. There is a slight asymmetry remaining which may be due to the presence of partially deuterated molecules (Fig. 2). Replacement of the p-hydrogen atom of benzyl alcohol by another group, such as a methyl group, does not affect the form of the ring CH bending mode described earlier, so that we may expect to observe the effect in the spectrum of p-methyl benzyl alcohol. Fig. 3(a) and (b) shows the effect at 1020 cm-i. The Raman spectrum of a saturated solution of p-methyl benzyl alcohol in carbon tetrachloride showed only a very weak broad band in this region, which is probably due to the CH,O mode. The absence of a Raman band which may be assigned to the aromatic CH bending mode is not., however, surprising. In the related symmetrical molecule, p-xylene, the mode is Raman inactive but is infrared active at 1020 cm-l; the corresponding mode in toluene is 10 cm-i higher. RANDLE and WHIFFEN [6]state that allpara-substituted benzene derivatives possess an infrared band at 1018 f 10 cm-l which they assign to the in-plane CH bending mode considered here. It is very probable that many para-substituted benzyl alcohol derivatives will be found to show the effect. In the infrared spectrum of benzyl alcohol vapor, Fig l(c), there is no indication of the vibrational interaction. This is not surprising in view of the different origins of band widths in the vapor and in condensed phases. Rotational isomerism in benxyl alcohol Internal rotation opposed by low potential barriers may occur about the C-O and C,,-C,, bonds in benzyl alcohol, and the unusually large width of the CH,O mode absorption band is very probably due to this molecular flexibility. Presumably, the interaction between the CH, and CH modes may occur only when the geometrical configuration is favorable, e.g. as in the configuration
this model has C,-symmetry
if the OH group is considered 132
as a unit.
Benzyl alcohol-an
unusual vibrational interaction
OKI and IWAMURA [7] have examined the doublet nature of the bands arising from the OH stretching modes of benzyl alcohol and its ring-substituted derivatives and have concluded that the favored configuration in benzyl alcohol in dilute solution is that drawn above with the hydrogen of the hydroxyl group interacting with the pi-electrons of the nearest ring carbon atom. Further evidence for the presence of rotational isomers is provided by Fig l(b), which shows the effect of dilution on the spectrum. While changes in the intermolecular hydrogen bonding may partly account for these spectral variations, it is also necessary to invoke rotational isomerism to account for all, e.g. near 1000 cm-l and near 740 cm-l. [7] M. OKI and H.
IWAMURA,
Bull. Chew
SW
Jnpnn
32, 955 (1959).
133
Benzyl alcohol-an
Printed in Northern Ireland
unusual vibrational interaction
J. C. EVANS Chemical Physics Research Laboratory The Dow Chemical Company, Midland, Michigan (Received 7 October1960) Abstract-Another example of an unusual interaction [I, 21 has been studied in detail in the vibrational spectra of benzyl alcohol. Infrared and Raman spectra of related molecules, including deuterated derivatives, were recorded to assist in the interpretation.
Introduction examples have been reported [ 1, 21 of unusual regions of distortion which occur in broad, infrared absorption bands of materials in condensed phases. It was proposed that these features are the result of interactions between fundamental vibrational modes and that such effects may be expected to occur whenever a relatively sharp energy level falls within the energy range of a much broader level. Comparisons of observed and calculated band envelopes were made [2]. In all of the examples previously presented the broad vibrational level involved was that of the NH, wagging mode in several aromatic amines; this mode yields a band of half-width about 150 cm-l in the 500-800 cm-l region and it is readily modified by change of molecular environment or by deuterium substitution. The other, sharp level involved was variable and in no case was a definite assignment to a particular vibrational mode possible. An example in which the interacting modes are an aromatic CH bending mode and a mode of the -CH,Ogroup has been found in benzyl alcohol. The results of a study of this case are presented here. SEVERAL
Experimental Commercial benzyl alcohol (b.p. 203-205°C) was used. C,D,.CH,OH was prepared by exchanging sodium benzoate with D,O in a sealed tube at 13O”C, in the presence of pre-reduced platinum oxide, for 24 hr [3], followed by conversion to benzoic acid and reduction of this to the alcohol using lithium aluminum hydride [a]. The exchange step was repeated twice before proceeding to the acid. The final product was estimated from its infrared spectrum to be approximately 85 per cent deuterium substituted, which was suffidiently pure for the present purpose. C,H,.CH,OD was made by direct exchange of benzyl alcohol with D,O. Infrared spectra were recorded using a prism-grating instrument designed and constructed in this laboratory [Fi]; spectral slit widths used were about 0.5 cm-l. [I] J. C. EVANS and N. WRIQHT, S~ecl~oochi~.Acta 16, 352 (1960). [2] [3] [4] [5]
J. C. EVANS, Spectrochim. Acta 16, 1382 (1960). W. G. BROWN and J. L. GAFCNETT, J. Am. Chem. SW 80, 5272 (1958). R.F. NYSTROM andW.G. BROWN, J. Am. Chem Sot. 69,254s (1947). L. W. HERSCHER,S~~C~~OC~~WS. Acta 15, 901 (1959).
1
129
J. C. EVANS
Raman spectra were recorded using a Hilger photoelectric instrument and 4358 A excitation (7 A/mm). Some spectra were also recorded photographically on the same instrument. Wavenumber values quoted in the text are believed to be better than &-2 cm-l.
Results and discussion The spectral data are illustrated in Figs. 1,2 and 3,in which the examples of the effect are pointed out by arrows. The most striking feature is the extremely sharp turning point at the peak of the narrow transmission region within the broad
Fig. l(a). The infrared spectrum (full line) and the Raman spectrum (dotted line) of liquid C,H,.CH,OH. The Raman spectrum is drawn on an arbitrery, linear, relative-intensity scele, while the optical-density scale refers to the infrared spectrum.
CM” Ce H;CH,OH
C,H,CH,OH _I
\
v1:‘; I
i I I “IL’
CS,
I 1200
I
I
_I
a 0
-
I’ 1
co.,:
-
Vapor
solution
I.0 -
,O l i, 1000
v1;
V
II
-
I’
I
1200
800
-CM-
Fig. l(b). Tbeinfraredspeotrumof CIH,.CH,OH in CS,; (i) 10 per cent solution in 0.106mm cell; (ii) 0.5 per cent solution in l-14-mm cell.
130
I
I
I I
I I
IO00
I
I 17.
800
c M-’
Fig. l(c). TheinfrctredspectrumofC,H,.CH,OH vapor in a lo-cm cell at 126°C with vapor pressure*; (i) that of the liquid at -lOO°C and (ii) at - 120°C.
Benzy
alcohol-an
unusual vibration&l i&era&ion
absorption band; this establishes that the effect is not merely the result of the overat non-in~r~t~~g bands. Theo~ti~l con~deratio~ [Z] indicated lap of two adj such sharp turning points, but the total effect observed in this case deviates somewhat from the predictions of simple theory. Whereas the effect observed here is reminiscent of the special case in whioh the sharp level coincides with the center of the broad level it is apparent from the figures that this condition is not satisfied. (0)
C,D;CH,OH II
11
11
II
(b)
II
O-
CM-’
Fig. 2. The infrared spectrum of a thin of liquid C*D~.~~*OH between KBr pl
Fig. 3. (a) The infrared speotrum of 8 liquid film of @-methy benzyl alcohol at ~70°C. (b) The ;aframd speutrkn of p-methyl benzyl alcohol in CS,; 5 per cent solution in O*l-mm toll.
However, the situation is probably complica~d by the finite intensity of the transition betweea the ground level and the sharp level in those molecules in which the interaction is weak. Fig. 1(a) shows that the effect in benzyl alcohol coincides with a strong Raman band at 1030 cm-l. All monosubstituted benzene derivatives show such a Raman band with the co~espondin~ sharp infrared band [ti] which are assigned to an inplane CH bending mode
The broad, strong band in the infrared spectrum of benzyl alcohol (half-width approx. 60 am-l) is very weak in the Raman spectrum. It undoub~dIy arises from a mode of the 4X,0group since it ap ars in the infrared spectra of 1R] R. R. RANDLE and D. H. WHIFFEN.
of Petroleum,
London (1955).
Inslituta ofPdmkumCo&rence,
x31
Lodon,
1954. p. 111. Institute
J.C.EVANS C,D,.CH,OH and of C,H,.CH,OD; in the infrared spectrum of the related molecule, C,H,.CH,SH, the broad band appears near 975 cm-l while the 1030 cm-l band is isolated and easily observed. Of the four CH, bending modes only one is to be expected in this general spectral region, the twisting mode. The high intensity of the broad band indicates however that the mode responsible is not a simple one and probably involves the entire -CH,O--group. A detailed knowledge of the nature of the mode is not essential for our purposes. The other band in this region-the very strong Raman band at 1004 cm-l is the phenyl ring breathing mode. It is invariably very weak in the infrared and, since it does not appear to interact with the CH,O mode it will not be discussed further. When the ring hydrogen atoms are replaced by deuterium the modes involving ring hydrogen motion are moved considerably to lower wavenumber and the effect should disappear. In accord with this, the effect is absent from the infrared spectrum of C,D,.CH,OH. There is a slight asymmetry remaining which may be due to the presence of partially deuterated molecules (Fig. 2). Replacement of the p-hydrogen atom of benzyl alcohol by another group, such as a methyl group, does not affect the form of the ring CH bending mode described earlier, so that we may expect to observe the effect in the spectrum of p-methyl benzyl alcohol. Fig. 3(a) and (b) shows the effect at 1020 cm-i. The Raman spectrum of a saturated solution of p-methyl benzyl alcohol in carbon tetrachloride showed only a very weak broad band in this region, which is probably due to the CH,O mode. The absence of a Raman band which may be assigned to the aromatic CH bending mode is not., however, surprising. In the related symmetrical molecule, p-xylene, the mode is Raman inactive but is infrared active at 1020 cm-l; the corresponding mode in toluene is 10 cm-i higher. RANDLE and WHIFFEN [6]state that allpara-substituted benzene derivatives possess an infrared band at 1018 f 10 cm-l which they assign to the in-plane CH bending mode considered here. It is very probable that many para-substituted benzyl alcohol derivatives will be found to show the effect. In the infrared spectrum of benzyl alcohol vapor, Fig l(c), there is no indication of the vibrational interaction. This is not surprising in view of the different origins of band widths in the vapor and in condensed phases. Rotational isomerism in benxyl alcohol Internal rotation opposed by low potential barriers may occur about the C-O and C,,-C,, bonds in benzyl alcohol, and the unusually large width of the CH,O mode absorption band is very probably due to this molecular flexibility. Presumably, the interaction between the CH, and CH modes may occur only when the geometrical configuration is favorable, e.g. as in the configuration
this model has C,-symmetry
if the OH group is considered 132
as a unit.
Benzyl alcohol-an
unusual vibrational interaction
OKI and IWAMURA [7] have examined the doublet nature of the bands arising from the OH stretching modes of benzyl alcohol and its ring-substituted derivatives and have concluded that the favored configuration in benzyl alcohol in dilute solution is that drawn above with the hydrogen of the hydroxyl group interacting with the pi-electrons of the nearest ring carbon atom. Further evidence for the presence of rotational isomers is provided by Fig l(b), which shows the effect of dilution on the spectrum. While changes in the intermolecular hydrogen bonding may partly account for these spectral variations, it is also necessary to invoke rotational isomerism to account for all, e.g. near 1000 cm-l and near 740 cm-l. [7] M. OKI and H.
IWAMURA,
Bull. Chew
SW
Jnpnn
32, 955 (1959).
133