Infrared spectrum of the methanol dimer in matrices. Temperature and irradiation effects in solid nitrogen

Four different open chain forms of methanol dimer have been identified in nitrogen matrices through temperature and IR irradiation effects. Each form is characterized by two OH stretching vibration frequencies, one (D1) close to the monomer frequency (3664 cm−1) and corresponding to the oxygen perturbed (electron donor) oscillator, and the other (D2) around 3500 cm−1 due to the hydrogen perturbed oscillator. Concentration ratios between two of these species (A: 3655 and 3519 cm−1; B: 3651 and 3504 cm−1) depends reversibly on temperature: the A form is predominant below 8 K while the B form is mainly observed above 10 K; this thermal process is fast. Moreover, the A or B concentration increases by IR irradiation at the expense of the C and D forms (C: 3659 and 3496 cm−1; D: 3654 and 3489 cm−1). This photoisomerization leads to a steady state with predominance of A or B, according to the temperature: its rate, relatively slow, has been quantitatively compared for OH and OD methanols. Since these processes are not observed for methanol in argon matrix, it is suggested that they are favored by the presence of nitrogen around the dimer.