Volumr 117, number 6 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA CHEMICAL PHYSICS LEXTERS 5 July 1985 FERMI RESONANCE AND YIR~~~NAL LINESHAPES OF THE CH, GROUP IN LIQUID MElTHANOL T-W. ZERDA, M. BRADLEY and J. JONAS Deparrnrenr of zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Chcmrrr-,. School of Cbemird Snencer, C/nic.ersrty of Ilhnors. 1209 W Calrfornra. Urbana. IL61801, USA Racivcd 8 April 1985 The CH slretchrng and bendmg vibralional Raman bands of liquid methanol have been mwured at temperawres zyxwvutsrqp rangmg from 0 IO 90°C and pressures from 1 bar LO 4 kbar. The coupled oscillator model is used to calculate frequencies and bandwtdths free of Femu resonance. Their dens&y and temperature dependenee is qualitatively explamed in termsof quickly varying repulsive forces_ The differences between the expenmental (with Fermi reonance) and ~lculated uncoupled {wnthous F+xml rsonanwe) bandwidths are found CO be m close agrctmem uwh the inwa.moleEular nxnsition model predicrions- l_ Introduction In a recent set of experiments performed in this laboratory [ 121 the Fermi-resonance and hydrogen- bondmg characteristics of hquid ammoma have been investigated_Methanol also exhibits these interactions as the Fermi couplmg occurs within the CH, group [3,4] . An important difference between the two sys- tems can be found in the location of the hydrogen bond relative to the atoms of the coupled modes; in ammonia the N-H bond experiences both H-bond and Fenm coupling, while in methanol the C-H bond undergoes the coupling, but the hydrogen bond is lo- cdlizcd on the O-H group. Since the Fermi mode coupIing depends on the anharmonic part of the intra- molecular potential [S] which hydrogen bonding 1s ‘known to effect [ 11, there will be significant differ- ences m the analysis for these two molecules. The goal of t&s paper is to analyze the high-pressure Raman data for the CH, vibration region in liquid methanol. As in our earlier work [ 1,2], separation of the line broadening due to Fermi resonance and that due to other effects is desired. Previously [ 1,3], two indepen- dent methods were used to acquire data with the Fermi resonance removed - experimentally via par!ial deuteration and theoretically using the coupled os- ci.lIator model. In the former method, two of the three protons in resonance were replaced with deuteriums breaking the coupling without sigruficantly altering the properties of the liquid- This was attempted for methanol by synthesizing the CD2HOH species_ How- ever, the C-H regmn for thus molecule shows two (or more) badly overlapped peaks thus obscuring the C-H bandshape. There are two techniques commonly used to model systems invoking Fermi resonance. ‘I&e simpler stan- dard theory [S] allows c~c~ation of uncoupled fre- quencies and anharmonicity coefficients but not line- widths. In the coupled oscillator model [6], a fitting procedure yields all of the desired information, XI- eluding uncoupled frequencies, anharmonicibes and bandwidths. The accuracy of these parameters denivecl from the fit was previously found to be excellent [ 1, 31 so they are taken to represent true uncoupled values here. 2. Experimental Methanol used in the high-pressureexpe,iments was obtained from Fischer Scientific with a stated purity of 99.9% CH,OH. The deuterated species CHDzOH was synthesized by Cambridge Isotopes and was reported to be 95% pure with the primary con- taminant being CHzE)OH. The CI-&OH was loaded di- rectly into the ceU. and pressurizingbellows while the 566 0 009-2614/W/% 03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)