Microwave measurements of the spectra and molecular structure for phthalic anhydride

The microwave rotational spectrum for phthalic anhydride (PhA) has been measured in the 4-14 GHz microwave region using a pulsed-beam Fourier transform (PBFT) Flygare-Balle type microwave spectrometer. Initially, the molecular structure was calculated using Gaussian 09 suite with mp2/6-311++G ^** basis and the calculations were used in predicting spectra for the measured isotopologues. The experimental rotational transition frequencies were measured and used to calculate the rotational and centrifugal distortion constants. The rotational constants for the normal isotopologue, four unique ¹³C substituted isotopologues and two ¹⁸O isotologues, were used in a least squares fit to determine nearly all structural parameters for this molecule. Since no substitutions were made at hydrogen sites, the calculated positions of the hydrogen atoms relative to the bonded carbon atoms were used in the structure determination. The rotational constants for the parent isotopologue were determined to be A = 1801.7622(9) MHz, B = 1191.71816(26) MHz, C = 717.44614(28) MHz. Small values for the centrifugal distortion constants were obtained; D_J = 0.0127 kHz, D_JK = 0.0652 kHz, and D_K = -0.099 kHz, indicating a fairly rigid structure. The structure of PhA is planar with a negative inertial defect of Δ = -0.154 amu Ų. Structural parameters from the mp2 and DFT calculations are in quite good agreement with measured parameters.