869
ASSOCIATION IN LIQUID METHANOL
A Vibrational Overtone Study of Association in Liquid Methanol by C. BourdBron, J.-J. PBron, and C. Sandorfy” DBpartement de Chimie, UniversitB de Montrbal, MontrBal, &&bee, Canada (Received September 97, 1071) Publication costs assisted by Universitb de Montrbal
An attempt has been made to demonstrate the presence of oligomers and monomers in liquid methanol, by a study of the OH stretching fundamental, and its first and second overtones. The key to the solution of this problem is the previously made observation that a decrease in the intensity of the polymer overtones gives the bands of the less associated species a chance to appear.
Introduction It was shown in the preceding paper1 that hydrogenbonded oligomers of sterically hindered alcohols subsist in solutions even at liquid nitrogen temperature. The identification of monomers and oligomers was seen to be facilitated in the region of the first overtone by a strong decrease in the intensity of the polymer bands. We now present results pertaining to an alcohol not affected by steric hindrance, methanol. For this molecule, the OH stretching fundamental and its first and second overtones were studied in the liquid state.
Interpretation of the Spectra (a) At room temperature in the fundamental region, we find only one nearly symmetrical broad band, centered a t 3340 cm-l, which is obviously due to the polymer. There is no evidence for the presence of either monomer or oligomers in this part of the spectrum of liquid methanol. (See the preceding paper for the sense given to the word “polymer”.) (b) The region of the first overtone (Figure l), also measured at room temperature, is much more complex. We find a broad feature containing a well defined peak a t about 6340 cm-l (A), with three apparent subbands (B,C,D). The peak a t 6340 em-’ corresponds t o the polymer; this is confirmed by the fact that a t low temperatures it becomes preponderant (curve 3, Figure 1). C and B have the right frequency for being oligomer bands (see the preceding paper), and D the monomer band. One inight argue that B is not really due to the OH stretching overtone of oligomers, but to combinations of the VCH VOH type, as was proposed by Fletcher and Heller.2*8 Curve 1in Figure 1 shows the spectrum of 0.05 M methanol in CClXF a t room temperature. (This spectrum was measured in a 10-em cell using the 1OX scale expansion of the Cary 17 instrument. The spectrum of the pure liquid was measured in a 0.2-em cell so that the products of concentration by optical path were approximately the same in both cases). Under these conditions the monomer band, a t about 7135 cm-l (vvhich has a peak absorbance of 4.7 on the scale of Figure 1) is preponderant, and a few weak
+
bands are found between 6500 and 6400 em-’. These bands are absent in the spectrum of both CH30D and CD30H, so they must be of the OH CH, type. The OH bands entering these combinations are due to the free OH groups, however, and are much too weak to interfere with the observation of the B band (Figure 1). Clearly, then, oligomer bands are present i n the first overtone region. As for the D band, the contribution of free OH groups to its observed intensity is hard to estimate because of possible CH and C D absorption in the 7100-cm-l region. We now have to examine the possibility that, as proposed by Luck and Ditter,6 the band a t 6340 cm-1 is due to a combination of the OH CH3type, involving the polymer OH stretching frequency. We therefore measured the spectrum of liquid CDIOH. At room temperature, observation is made difficult because of interference from the 3vCD bands. At -loo”, as polymers become preponderant, we find a strong band at the same frequency as for CH30H. This is seen even more clearly in a solution of CD30H in a 1:1mixture of CCZF and C2F4Br2at - 190”. The same observation was made previously by Asselin and Sandorfye in the case of 2-propanol-d, and 2-propanol. Thus it is confirmed that the band at 6340 em-’ is due to the polymer. Whereas a t the level of the fundamental only polymer bands are seen, monomer and oligomer bands appear at the first overtone level, the polymer still being the strongest . (e) In order to interpret the second overtone region, we have to eliminate any possible absorption due to the methyl group. To do this we measured the spectrum of CH3OH against CH30D. Since OD does not absorb
+
+
(1) C.BourdBron, J.-J. PBron, and C. Sandorfy, J. Phys. Chem., 76, 864 (1972). (2) A.N.Fletcher and C . A. Heller, J . Phgs. Chem., 71,3472 (1967). (3) A. N.Fletcher and C . A. Heller, ibid., 72, 1839 (1968). (4) R.F. Goddu and D. A. Delker, Anal. Chem., 32, 140 (1960). (5) W.A. P.Luck and W. Ditter, Ber. Bunsengee. Phgs. Chem., 72, 365 (1968). (6) M.Asselin and C . Sandorfy, J. Chem. Phys., 52, 6130 (1970).
The Journal of Physical Chemistry, Vol. 76, No. 6,1972
870
C. BOURDERON, J.-J. PERON, AND C. SANDORFY -1Ocm CHaOH ...lOcm C H ~ O H O ~ ~ ~ ~ CHsOD SIIOC~ lOcm CD30H
0.7-
Cm-'
Figure 1. The first OH stretching overtone of liquid methanol. Curve 1: 0.05 A t solution in CC13F a t room temperature in a IO-cm cell using the lox scale expansion. Curve 2: liquid at room temperature in a 0.2-cm cell. Curve 3: 0.24 M solution in a 1:1 mixture of CC1,F and CtF4Br2at -190". Cell length: 10 cm, using a 2X scale expansion.
in this part of the spectrum, all the observed absorption must be due to the OH group. The result was curve 2 in Figure 2. It is very different from curve 1 which belongs to liquid methanol. Then, we went on measuring the spectrum of CD30H and obtained curve 3. The resemblance between curves 2 and 3 shows that they are both due to OH absorption. I t is seen that in the second overtone region, the center of gravity is in the oligomer part (C). The free species is represented by a strong shoulder (D) at its high frequency side and there are two more bands at lower frequencies (A and B) which must correspond to more highly associated species (A surely being due to the polymer). It may be difficult to believe that the broad C band corresponds to oligomers and the polymers give the weaker A and perhaps B bands. Thus we measured the spectrum at -100", and found a spectacular decrease in the intensity of C, while the intensity of A increased (Figure 3). Since the lowering of temperature certainly favors the polymer, the C band cannot be the polymer band.
Discussion We assign the 3340-cm-l band to the polymer fundamental, 6340 cm-l to its first overtone, and 9220 cm-l to its second overtone. The last assignment takes into account the effect of CHa absorption (see above), and is different from the one proposed by Luck and Ditter6 who put it at 9760 cm-I. From these frequencies we obtain the following (approximate) anharmonicity constants
'02 03 XZ3= - '= 100 cm-1 2 3
The latter value is of the same order as the one obThe Journal of Physical Chemistry, Vol. 76,N o . 6,1972
IlOOO
10500
10000
9000
9500
Cm-'
Figure 2. The second OH stretching overtone of liquid methanol. Curve 1: liquid in a 10-cm cell. Curve 2: CHIOH compensated by CHIOD in 10-cm cells. Curve 3: liquid CDaOH in a 1-cm cell using the lox scale expansion.
i
07t
,
0 f} /r'
,;;/, 10500
I
1.
,,-.J .
