Microwave spectrum and the gas phase structure of phthalimide

Abstract

The microwave spectrum of phthalimide (PhI) was measured in the 4.8–9.5GHz range using a Flygare-Balle type, pulsed-beam Fourier transform microwave spectrometer. Rotational transitions were measured for the parent and all unique single 13C substituted isotopologues. The rotational (MHz), centrifugal distortion (kHz), and quadrupole coupling constants (MHz) were determined for the parent to be A=1745.6655(10), B=1199.3309(6), C=711.0864(3), DJ=0.012(7), DJK=−0.05(9), 1.5χaa=2.719(11), and 0.25(χbb−χcc)=1.236(3). Using the measured rotational constants of the isotopologues, a nonlinear least squares fit was performed to obtain the best fit gas phase structure. The inertial defect is Δ=−0.175amuÅ2, indicating a planar structure for PhI.Calculations using B3LYP/aug-cc-pVQZ provided rotational constants which are much closer to the experimental values compared to the MP2/6-311++G∗∗ calculated values.