Molecular structure and vibrational spectrum of N-methylpyrrolidine betaine hydrogen chloride: experimental and DFT study

The geometry, frequency and intensity of the vibrational bands of N-methylp yrrolidine betaine hydrogen chloride (MPRBET . HCl) and its deutero derivat ive (MPRBET DCL) were obtained by the density functional theory (DFT) with the BLYP and B3LYP functionals and using the 6-31G(d,p), 6-31 + + G(d,p) an d mixed basis sets. The calculated frequencies are compared with the solid IR spectra. All of the measured IR bands were interpreted in tens of the ca lculated vibrational modes. Most computed bands are predicted to be at high er wavenumbers than the experimental bands. The best agreement was found be tween the experimental spectrum and the 'harmonic oscillator' spectrum comp uted at the BLYP/6-31G(d,p) level of theory; the r.m.s. deviations between the experimental and calculated harmonic frequencies are 37 cm(-1) for all bands and 18 cm(-1) for all but without the v(OH) band. The calculated O .. . Cl- distance is longer by ca. 0.08 Angstrom than the X-ray value and the calculated frequency of the vOH vibration is overestimated by ca. 200 cm(-1 ). (C) 2000 Elsevier Science B.V. All rights reserved.