Solvation of cyanoalkanes [CH3CN and (CH3)3CCN]. An infrared and nuclear magnetic resonance study

Abstract

The CN stretching band (ν₂) has been studied for dilute solutions of CH₃CN and (CH₃)₃CCN in a range of aprotic and protic solvents. The former induce a low-frequency shift, whereas the latter induce a high-frequency shift relative to dilute solutions in hexane. In both cases there is a large increase in oscillator strength with increasing shift. This is the first example of a solvent that has an absorption band displaying such a dichotomy, the normal behaviour being a progressive low-frequency shift on going via aprotic to protic media. In contrast such ‘normal’ behaviour is observed in the n.m.r. spectra for ¹⁴N shifts, but the ¹³C (CN) shifts are small and seem to be random. In contrast to our previous studies of ‘probe’ molecules the ν₂ bands for solutions in water are almost identical to one of the bands in methanol; however, the band in water is a single feature, whilst that in methanol is a doublet, the low-frequency feature being close to the unsolvated region. The interpretation is that MeCN in water is fully monosolvated (hydrogen-bonded), whilst in MeOH it is only ca. 50% monosolvated. However, the effects of temperature changes and studies of mixed water–aprotic solvent systems suggest that this may not be correct, and the possibility that MeCN forms two very weak hydrogen bonds in water is also considered.

The methanol doublets are well defined at low temperatures (–50 °C) but resolution is lost on warming. At ca. 50 °C there is only one symmetrical band. For mixed water–aprotic solvent systems, the band remains a narrow singlet throughout the whole mole fraction range, there being no indication of twin bands for hydrogen-bonded and non-hydrogen-bonded units, in contrast with the results for methanol at low temperatures, and our normal experience with other probe molecules. One explanation is that there is rapid equilibrium between hydrogen-bonded and non-hydrogen-bonded units which is fast on the i.r. timescale. Results for other mixed-solvent systems are also reported.

We have attempted to use changes in the first and second overtone O—H stretch bands for HOD in D₂O on adding MeCN to obtain a measure of the number of hydrogen bonds. This is at best only qualitative because of the proximity of the O—H band for solvated MeCN and the ‘(OH)_free’ band for water. However, for MeCN in Me₃COH the (OH)_free and OH---(NCMe) bands are clearly resolved. The results support the concept that MeCN is dihydrated in water. A less extensive study has been made for 2-cyano-2-methylpropane (Me₃CCN) for comparative purposes. The results are broadly similar, the major difference being a reduction in the extent of hydrogen bonding in methanolic solutions.