On the “Anomalous” Temperature Dependence of the FIR‐Absorption Band of Acetonitrile

Abstract

Absorptionsspektren, Infrarot / Fliissigkeiten / Quantenstatistik 1 Statistische Mechanik / Zwischenrnolekulare Wechselwirkungen An anomalous red-shift of the FIR-absorption band of CH,CN in n-heptane with increasing temperature had been reported in Ref. [l]. This result is experimentally confirmed. We also observed the same effect in some other solvents (CC14, CS,, and cyclohexane). In the case of n-heptane the temperature dependence of the overall absorption intensity attains a maximum within the temperature range under examination. The theoretical interpretation of the anomalous red-shift as given in previous papers is mentioned. I. IntroductionAn anomalous "red-shift'' of the FIR-absorption band of dilute solutions of acetonitrile (CH3CN) in n-heptane has been observed at first by Kroon and van der Elsken [l]. In this case the FIR band shifts continuously to lower frequencies with increasing temperature. As these authors pointed out: "This now is contrary to what one could expect for a pure rotation band of a gas and is also in disagreement with existing theories (of R. G. Gordon, cf. Refs.[ 2 ] ) for rotation bands in liquids"; cf. Ref.[l], p. 287.As a matter of fact this effect also seems to contradict the universal validity to the Boltzmann distribution law in the case of liquids [3, 41.The aforementioned "anti-Boltzmann" effect of acetonitrile has been confirmed Experimental and ResultsFigs. 1 and 2 show the results of our investigations. We used acetonitrile (beeing 'uvasol', for spectroscopy, Fa. Merck) as the dipolar solute in different nonpolar solvents such as CCI,, CS2, cyclohexane and n-heptane, and we varied the concentrations within the range of 0.2 mol/l down to 0.01 mol/l. Using those higher concentrations there has been much trouble with the solubility of acetonitrile at low temperatures. All absorption spectra were recordered on a Bruker Fourier Transform spectrometer IFS 11 3v.All solvents cause significant bandshifts in case of temperature variation. But those band shifts do not behave Boltzmann-like. Variation of temperature from 253 K to 333 K in n-heptane results in a shift of the maximum from 56 cm-' to 40 cm-'; using cyclohexane the maximum shifts from 55 cm-' to 45 cm-' by temperature variation from 294 K to 333 K.In the case of solvents with electric multipoles such as CCI, or CS, the maximum of absorption does always appear at higher wavenumbers. In CCI, the shift is going from 78 cm-' to 62 cm-' (258 K to 333 K) and in CS, it is going from 76 cm-' to 68 cm-' (246 K to 293 K).Furthermore one can see from Table 1 that 6 (integral extinction coefficient) decreases with increasing temperature. But there is one exception to be mentioned. The integral extinction coefficients of acetonitrile in n-heptane exhibits a maximum value around 270 K (see also Fig. 2).Below 270 K the overall intensity rises with increasing temperature whereas above 270 K it falls down rapidly. Such behaviour of the overall intensity has not been found in all other solvents under investigation. The value of the i...