Chemical Physics ELSEVIER Chemical Physics 208 (1996) 391-401 Fourier transform microwave study on 2-methyloxetane and 3-methyloxetane Jose L. Alonso *, Alberto Lesarri, Santiago Mata, Juan C. L6pez, Jens-Uwe Grabow 1, Helmut Dreizler 2 Departamento de Qufmica Ffsica, Facultad de Ciencias, Universidad de Valladolid, E-47005 Valladolid, Spain Received 2 January 1996 Abstract The rotational spectra of 2-methyloxetane and 3-methyloxetane have been reinvestigated using Fourier transform microwave spectroscopy (FTMW). A,E splittings due to internal rotation of the methyl group have been observed in the ground and several ring-puckering and methyl torsion excited states for both molecules• Internal rotation barriers of V 3 = 3.332(14) kcal mo1-1 (13.941(58) El mo1-1) for 2-methyloxetane and V3= 3.284(12) kcal mol -l (13.740(50) kJ mol-~) for 3-methyloxetane have been determined from the ground vibrational state spectra. Furthermore, the rotational, quartic and some higher-order centrifugal distortion constants are reported. The experimental set-up of a waveguide Fourier transform microwave spectrometer is also presented. 1. Introduction In previous papers we reported an analysis on the rotational spectra of 2-methyloxetane CHs-CH-O-CH2-CH2, 2MO I I and 3-methyloxetane CHr'CH-CH2-O-CH2, 3MO l, l * Corresponding author. ~ax: + 34 83 423264. Permanent address: Institut fiir Physikalische Chemie der Universif~it Kiel, OIshansenstrasse 40, D-24098 Kiel, Germany. 2 Permanent address: Institut flit Physikalische Chemie der Universit~it Kiel, OIshausenstrasse 40, D-24098 Kiel, Germany. • An asymmetric single minimum ring-puckering potential function with an equatorial equilibrium conformation was found for 2-methyloxetane [1]. For 3-methyloxetane a slightly asymmetric double mini- mum ring-puckering potential function similar to that of the parent molecule oxetane was proposed [2]. Because of the high barriers hindering internal rota- tion of the methyl group in both molecules no tor- sional fine structure was observed by using Stark spectroscopy. We have reinvestigated the spectra of 2MO and 3MO using Fourier transform microwave spec- troscopy (FTMW), which offers the advantages of both superior resolving power and potentially higher sensitivity compared to conventional Stark effect modulation spectroscopy. The present study was un- dertaken with the initial goal of exploiting these advantages to resolve the internal rotation fine struc- ture in the ground and excited states to obtain the barriers to internal rotation of the methyl group for 0301-0104/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. Pll S0301-0104(96)00090-0