The room-temperature cavity ringdown absorption spectra of 2-cyclopenten-1-one (2CP) and deuterated derivatives were recorded near 385 nm. The very weak ( < 1 M -1 cm -1 ) band system in this region is due to the T 1 r S 0 electronic transition, where T 1 is the lowest-energy 3 (n,π*) state. The origin band was observed at 25 963.55(7) cm -1 for the undeuterated molecule and at 25 959.38(7) and 25 956.18(7) cm -1 for 2CP-5-d 1 and 2CP-5,5-d 2 , respectively. For the -d 0 isotopomer, about 50 vibronic transitions have been assigned in a region from -500 to +500 cm -1 relative to the origin band. Nearly every corresponding assignment was made in the -d 2 spectrum. Several excited-state fundamentals have been determined for the d 0 /d 2 isotopomers, including ring-twisting (ν′ 29 ) 238.9/227.8 cm -1 ), out-of-plane carbonyl deformation (ν′ 28 ) 431.8/420.3 cm -1 ), and in-plane carbonyl deformation (ν′ 19 ) 346.2/330.2 cm -1 ). The ring-bending (ν′ 30 ) levels for the T 1 state were determined to be at 36.5, 118.9, 213.7, 324.5, and 446.4 cm -1 for the undeuterated molecule. These drop to 29.7, 101.9, 184.8, 280.5, and 385.6 cm -1 for the -d 2 molecule. A potential-energy function of the form V ) ax 4 + bx 2 was fit to the ring-bending levels for each isotopic species. The fitting procedure utilized a kinetic-energy expansion that was calculated based on the structure obtained for the triplet state from density functional calculations. The barrier to planarity, determined from the best-fitting potentialenergy functions for the -d 0 , -d 1 , and -d 2 species, ranges from 42.0 to 43.5 cm -1 . In the T 1 state, electron repulsion resulting from the spin flip favors nonplanarity. The S 0 and S 1 states have planar structures that are stabilized by conjugation.

Several rotational levels in the lowest excited bending state of HeHCN have been observed at hyperfine resolution by electric resonance spectroscopy near 100 GHz. The observed transitions correlate to the j = 1 -0 transition in the limit of free internal rotation. The ground state has been characterized by using millimeter wavehicrowave double resonance. One-photon transitions in the ground state are not observable using electric resonance, due to poor focusing of the nominally j = 0 levels. Ground-state J = 1 + 0 and J = 2 + 1 transitions were measured at 15 893.6108(41) and 31 325.2443(82) MHz, respectively. Quadrupole coupling constants eqJQ were determined to be 0.11 18( 15) MHz for J = 1 and 0.199( 12) MHz for J = 2. We have calculated rovibrational energies and wave functions arising from an ab initio intermolecular potential, calculated at the MP4 level using a large basis set containing bond functions. The potential is characterized by a well depth of 25 cm-' at the centers of mass separation R = 4.27 A. The global minimum occurs at the collinear He-H-C-N configuration, and the minimum energy rises monotonically, with large angular-radial coupling, as the HCN orientation angle 8 increases from 0 to n. Calculated and observed transition frequencies, including hyperfine structure, agree to within 10%. We have used the calculated Coriolis interaction energy to deperturb the measured ground-state spectroscopic constants. This procedure permits estimates of vibrationally averaged structural parameters. We find, for the ground state, (R-2)-"2 = 4.23 A. Very large amplitude radial motion results from zero-point energy that is 75% of the 25-cm-' well depth. The hyperfine data reflect very weak anisotropy in the potential, with (P2(cosO)) = 0.092 (J = 1) and (P2(cos8)) = 0.1 15 (J = 2). These values are very close to (P~(cos8)) = 0, characteristic of a free internal rotor. The centrifugal distortion of eqJQ indicates that, as in the other rare gas-HCN complexes, significant angular-radial coupling causes the HCN to align with the intermolecular axis in the rotating complex.

The intermolecular potential surfaces for ArHCN and ArHCCH are computed by Mo/ller–Plesset perturbation theory at the fourth-order approximations (MP4) with a large basis set containing bond functions. Rovibrational energies and spectroscopic constants of the two systems are computed from the intermolecular potentials using the collocation method. The intermolecular potential for ArHCN at the MP4 level has a single minimum at the collinear Ar−H−C−N configuration (R=4.56 Å, θ=0°) with a minimum potential energy of Vm=−135.9 cm−1. The bending frequencies, rotational constants, and centrifugal distortion constants of ArHCN and ArDCN calculated using the MP4 potential are in good agreement with experiment. Rovibrational energies with J=0 through 6 arising from j=0 and j=1 levels of HCN are calculated and compared with the experimental transition frequencies. The intermolecular potential surface for ArHCCH has a symmetric double minimum near the T-shaped configuration. The minimum positions at the MP4 level are (R=4.05 Å, θ=60° and 120°) and the minimum potential energy is Vm=−110.9 cm−1. The rotational constants and bending frequency of ArHCCH arising from the MP4 potential are calculated and compared with experiment. The anisotropy of the MP4 potential is slightly underestimated. The effects of monomer bending vibration on the ArHCN and ArHCCH potentials are studied by additional calculations. The potential anisotropy of ArHCN decreases, whereas that of ArHCCH increases as the monomer vibration is taken into account. This might be partially responsible for the discrepancies between the theoretical predictions and experiment.