THE EFFEXT OF PRESSURE ON THE VIBRATIONAL FREQUENCY OF BONDS CONTAINING HYDROGEN * BY A. M. BENSON AND H. G. DRICKAMER Dept. of Chemistry and Chemical Engineering, University of Illinois, Urbana, Illinois Received 30th April, 1956 Using a technique described earlier, measurements have been of the effect of pressure on the vibrational frequency of a series of chemical bonds mostly containing hydrogen. The systems were studied in CS2 and CFC13. The shifts vary from the “red shifts” resulting from van der Waals’ forces in OH to a definite “ blue shift ” for the antisym- metric CH2 vibration. In general the repulsive forces are effective at lower relative densi- ties in CFCI3 than in -2. While the data are not yet extensive, certain regularities are noted which should have theoretical significance.A possible application of the pressure coefficient of the frequency to the identification of molecular structure is noted. In previous papersl.2 a method for studying infra-red spectra in solutions under pressures to 12,000 atm, has been described and applied to the OH stretching vibration of n-butanol in a series of solvents. A “ red-shift ” (to lower frequencies) was obtained which varied approximately as p2/r6 and which was roughly propor- tional to the polarkability of the most polarizable bond in the solvent. This shift seems clearly due to the attractive interaction between the solvent, or at least the more polarizable bonds of the solvent, and the hydrogen in the OH bond. At some density the controlling interaction must become repulsion rather than attraction.Since this critical density should be different for different bonds, we have extended our investigation to include a series of other bonds mostly involving hydrogen. The apparatus used was essentially the same as described in the previous papers. The spectrometer was a Perkin-Elmer single-beam double-pass type, employing LiF optics. The systems studied are shown below. TABLE 1 bond molecule solvent concentration vibration NH SH CH CH CH2 CH2 OH OH CF C6H5NH2 n-C3H7SH CHC13 CHCl3 CH2CI2 CHzCl;! n-C4HgOH CfjH5OH CFCl3 CFC13 -2 cs2 CFCI3 cs2 CFCl3 CFCl3 a 2 CFC13 0 4 % vNH stretch (s and a) 5 % vSH stretch 3 %,5 % vCH stretch vCH stretch 6 % 5 % vCH stretch (s and a) 5 % vCH stretch (s and a) 0 7 % vOH stretch 0 6 mg/d vOH stretch 100 % 3vCF stretch The chemicals used were generally the best available commercially, in some cases purified further by distillation.The runs we= made in a room maintained at constant temperature and each point was re-run 3 or 4 times and the average shift taken. While the accuracy vaned with the dispersion and with other factors, the shifts were reproducible generally to a fraction of a wave number. * This work was supported in part by the U.S. Atomic Energy Commission. 3940 VIBRATIONAL FREQUENCY OF BONDS No density data were available for CFC13 as a function of pressure. We have made some crude p , w, t measurements at 24-25’ C and supplemented these 1 I I 3 9 CF s t r e t c h I I 1 1.3 -201 I *o 1 . 1 1.2 FIG. 1. from Bridgman’s data on CCl4 and CHBr3. The relative den- sities used in the plots were: TABLE 2.-RELATIVE DENSITY OF CFC13 P (atm) P I P 0 1 l*OOo 500 1.053 1 ,OOo 1.092 ~ , O O o 1.151 4,000 1.215 6YOOo 1.255 8YOOo 1.285 10,OOo (extrapolated) 1.305 The results are shown in fig.1-6. A really satisfactory theoretical analysis was not possible before the deadline for papers, but the salient features will be indicated. From fig. 1 and 2 it can be seen that the CF and OH stretch- ing frequencies shift to lower frequencies (“ red shift ”) with a roughly linear density dependence. A somewhat better correlation is obtained by plotting against p2. If only inter- action with nearest neighbours is considered a p2/r6 dependence would be pre- dicted. If the surroundings are treated as a continuum, a pJr3 dependence is predicted.The experimental results lie between, but nearer the former case. The magnitude of the shift for n-C4HgOH in CFC13 is consistent with the polar- izability of the CCl bond (see Fishman and Drickamer 1). The red shift for phenol in CS2 is very close to that obtained previously for n-C4HgOH and CH30H in CS2 for all except the two highest pressures. This is apparently due to phenol freezing out at these pressures. In general for these dilute solutions there seems to be very little effect of the size of the group attached to the OH, although there was some indication in our earlier data for t-C4HgOH of shielding of the OH by the adjacent CH3 groups. It is relatively easy to show that the magnitude of the shift should be propor- tional to the number of quantum jumps between the initial and the final state, i.e.to the number of the overtone. Since we measured the second overtone (34 for CF, the fundamental would show a maximum shift of about five wave numbers at the highest density. This is consistent with the relatively weak attractive forces in fluorocarbons, as evidenced by low cohesive energy and large molar volume. In fig. 3-5 we show the shifts with pressure of the CH and CH2 stretching vibrations for CHCl3 and CH2Cl2 in CS2 and in CFC13. Here, for the first time, one obtains “blue shifts”, i.e. to higher frequency, with increasing density at pressures well below 10,OOO atm. The CH stretching vibration in CHCl3 and the symmetrical CH2 stretching vibration in CH2C12 behave in a qualitatively similar manner.Both are of the same symmetry type (A1). In CS2 they both give a red shift throughout the entire pressure range. In CFCl3 both show a small red shift at low pressures followed by a small but distinct blue shift at high density. Apparently theA . M . BENSON AND H. G . DRXCKAMER 41 repulsive forces become significant at lower relative densities in CFC13 than in CS2. Of course, since the molar volumes and other parameters are not identical for the two solvents at 1 atm, we are not measuring our relative densities from the same fiducial conditions for the two solvents. I C 5 A 3 (cm": C - 5 -- - - 0 . 7 5 % n BuOH i n CSZ 0 7 5 x M e O H in CS, 1.0 1.1 1.2 1.3 (f 1 Po FIG. 2. C H s t t t t c h ( a n t . ) CH, CI,42 VIBRATIONAL FREQUENCY OF BONDS The antisymmetric (type Bj) CH2 vibration in CH2Cl2 shows a greater im- portance for repulsive forces at any given density than does the symmetric vibra- tion.This is true in both solvents. It is interesting to note that the difference between the shifts for the symmetric and antisymmetric vibrations are nearly the same at the same relative density in each solvent. For this vibration also repulsive effects are more important in CFC13 than in CS2. FIG. 5. It should be noted that the CF stretching vibration shows no apparent effect of repulsive forces even at the highest pressures, whereas the CH stretch in CHC13 shows a distinct blue shift. One might expect that the electrons clustered on the fluorine would lead to repulsion at relatively low densities. Apparently the attractive potential for CF is rather shallow, but the repulsive potential rises steeply only at quite small intermolecular distances.A. M.BENSON AND H. G . DRICKAMER 43 Fig. 6 shows the results for the NH2 vibrations of aniline in CFC13 and for the SH vibration of n-C3H7SH in CS2. The N H 2 vibrations show somewhat less effect of repulsion in the pressure range than do the CH2 vibrations. As might be expected, the effect lies between that for OH and for CH2. It may be noted that there is no significant difference between the symmetric and antisymmetric N H 2 vibrations such as was found for CH2. This point requires further in- vestigation. 3 SH stretch 5X.n-PrSH i n CS2 --I O - 1 - ' / - - 2 - V N H stretch(ant.) - 0-4%@-NH inCFC13 I ! I I - 2 ~ N H stretch (sym.) 0.4% @-NH i n C F C l j The SH stretching vibration shows the effect of repulsion at somewhat lower pressures than does the OH, as might possibly be expected. The results obtained to date raise several interesting theoretical questions which we plan to treat, along with more extensive results, in a future paper, One interesting possible application of the work 3 lies in the field of identification of molecular structure. If each vibration has not only a characteristic frequency but also a characteristic pressure coefficient, by running spectra at several pressures, it might be possible to resolve many ambiguities which arise in the usual 1-atm spectrum. This work was supported in part by the U.S. Atomic Energy Commission. A. M. Benson would like to acknowledge a fellowship from the National Science Foundation. 1 Fishman and Drickamer, J. Chem. Physics, in press. 2 Fishman and Drickamer, Anal. Chem., May, 1956. 3 Douglas Applequist, private communication.