Stephen Drucker

Ab initio potential energy surface and dynamics of He–CO

The potential energy surface for the He–CO van der Waals interaction is calculated by the supermolecular approach using fourth‐order Mo/ller–Plesset perturbation theory (MP4) with a large basis set containing bond functions. The rovibrational energies of He–CO are then calculated by the collocation method. Our ab initio surface has a single near T‐shaped minimum (Rm=3.49 A, θ=120°, Vm=−20.32 cm−1), in agreement with a recent experimental potential (R=3.394 A, θ=121.3°, Vm=−22.91 cm−1), determined from high‐resolution spectroscopic measurements, but significantly different from a previously published ab initio surface for this system. The calculated rovibrational energies are in good agreement with experiment. The explicit dependence of the intermolecular potential on the CO bond distance is also studied by MP4 calculations, and the results confirm the experimental observation that the intermolecular coordinates are approximately uncoupled from the CO bond distance.

Intermolecular potentials and rovibrational energy levels of the Ar complexes with HCN and HCCH

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 A, 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 ar...

Spectroscopic characterization of the lowest Π and Σ bending states of ArHCN

The lowest excited bending states, Σ1 and Π1, of the ArHCN complex have been measured by millimeter‐wave electric resonance optothermal spectroscopy. The principal molecular constants determined for the Σ1 state are ν0=164 890.790(12) MHz; B=1958.8571(37) MHz; D=−0.075 23(29) MHz; eqaaQ=0.825(27) MHz; and μa=−0.521(30) D. For the Π1 state, the constants are ν0=181 984.4126(47) MHz; B=2031.3624(17) MHz; D=0.153 35(16) MHz; eqaaQ=0.904(11) MHz; and μa=0.273 02(63) D. The leading Σ1–Π1 coupling constants are the Coriolis coefficient β0=1016.998(13) MHz and the transition dipole moment μb=2.2535(57) D. The rotational constants for the two bending states indicate that the average separation between the argon and the HCN center of mass contracts by roughly 0.5 A compared to the linear ground state. This is consistent with the nearly T‐shaped average geometry for each state established by analysis of the dipole moments and quadrupole coupling constants. Agreement between this work and prior theory confirms attri...

High resolution spectrum of the v=1 Π state of ArHCN

The molecular complex ArHCN shows extreme nonrigidity in the rotational spectrum of the ground vibrational state.im3 This state appears to be that of a linear molecule with very large zero point amplitude in the y5 van der Waals bend. The most dramatic evidence for this interpretation is seen in the centrifugal distortion constants, D(ArHCN) = 172.3 kHz and D(ArDCN) = 101.8 kHz.’ These constants are an order of magnitude larger than those of the prototypical species ArHCl, and their ratio is also an order of magnitude larger than the D(ArHCl)/ D(ArDC1) ratio.4 This indicates that the quasiadiabatic separation usually effected between the bending and stretching coordinates is not possible because of a large angular-radial coupling. Regarding the HCN as a rigid rod, the dynamics of ArHCN are usually treated using the standard Jacobi coordinate system of centers of mass separation R and internal orientation 8. The minimum energy of the linear configuration (8 = 0”) has a larger center of mass separation than the “tee” shaped configuration (6 = 90”)) which the strong angular-radial coupling argues is nearly isoenergetic. The nonrigidity is a consequence of motion along a path between these two structures. Several experimental and theoretical studies of this system have focused on the properties of the excited vibrational states of ArHCN.2T5P6P8 In addition, semiempirical treatments have been designed to fit the bizarre consequences of the nonrigidity of the ground state.7 These lead to predictions of the properties of the excited bending states. In this letter, we present observations of the first excited Il bending state by millimeter spectroscopy. The experimental apparatus is very similar to the spectrometer of Fraser et aL9 It has three separate diffusionpumped chambers. In the first chamber, a molecular beam is generated by adiabatic expansion and collimated with a 1 .O mm diam skimmer. Transitions are induced in a second chamber of 15 cm length. Microwave radiation from 2-300 GHz is focused into this chamber by means of a Teflon lens. The third chamber is occupied by a 100 cm long hexapole focuser with 6.35 mm diam rods and 11.4 mmdiam central aperture. Nominal focusing voltages of 2-35 kV are applied. An x-y positioned 1.6 mm diam beamstop blocks the on-axis component of the molecular beam from entering the third chamber, preventing the nonfocused carrier gas from reaching the detector. The detector is a liquid helium-cooled bolometer (Infrared Laboratories), mounted on a vacuum-sealed x-y positioner. In the present experiment, the beam of ArHCN is generated by a room temperature expansion of 2-3 atm. of i% HCN in argon through a circular nozzle of approximately 40~ diam. Radiation near 6 cm ’ is generated by doubling the output of a 3 cm ’ klystron, phase locked to a harmonic of a frequency synthesizer.” The frequency is modulated by applying a 200 Hz square wave to the phase-lock loop reference oscillator. Resonances P(2)-P(6), Q(l)-Q(5), and R(O)-R(2) are observed. A typical spectrum is shown in Fig. 1. Nearly all of the resonances were observed as enhancement of the bolometer signal, indicating that both halves of the I-doublet contain components with positive Stark coefficients. The molecular carrier and lower state rotational quantum numbers are ascertained by double resonance, using the precisely determined rotational transition frequencies of the ground vibrational state. The term values of the observed levels and their effective quadrupole coupling constants are given in Table I. For the Q branch lines, these are determined from the following energy expression:

Lowest triplet (n, π*) electronic state of acrolein: Determination of structural parameters by cavity ringdown spectroscopy and quantum-chemical methods

The cavity ringdown absorption spectrum of acrolein (propenal, CH(2)=CH-CH=O) was recorded near 412 nm, under bulk-gas conditions at room temperature and in a free-jet expansion. The measured spectral region includes the 0(0)(0) band of the T(1)(n, π*) ← S(0) system. We analyzed the 0(0)(0) rotational contour by using the STROTA computer program [R. H. Judge et al., J. Chem. Phys. 103, 5343 (1995)], which incorporates an asymmetric rotor Hamiltonian for simulating and fitting singlet-triplet spectra. We used the program to fit T(1)(n, π*) inertial constants to the room-temperature contour. The determined values (cm(-1)), with 2σ confidence intervals, are A = 1.662 ± 0.003, B = 0.1485 ± 0.0006, C = 0.1363 ± 0.0004. Linewidth analysis of the jet-cooled spectrum yielded a value of 14 ± 2 ps for the lifetime of isolated acrolein molecules in the T(1)(n, π*), v = 0 state. We discuss the observed lifetime in the context of previous computational work on acrolein photochemistry. The spectroscopically derived inertial constants for the T(1)(n, π*) state were used to benchmark a variety of computational methods. One focus was on complete active space methods, such as complete active space self-consistent field (CASSCF) and second-order perturbation theory with a CASSCF reference function (CASPT2), which are applicable to excited states. We also examined the equation-of-motion coupled-cluster and time-dependent density function theory excited-state methods, and finally unrestricted ground-state techniques, including unrestricted density functional theory and unrestricted coupled-cluster theory with single and double and perturbative triple excitations. For each of the above methods, we or others [O. S. Bokareva et al., Int. J. Quantum Chem. 108, 2719 (2008)] used a triple zeta-quality basis set to optimize the T(1)(n, π*) geometry of acrolein. We find that the multiconfigurational methods provide the best agreement with fitted inertial constants, while the economical unrestricted Perdew-Burke-Ernzerhof exchange-correlation hybrid functional (UPBE0) technique performs nearly as well.

Lowest triplet (n,π*) state of 2-cyclohexen-1-one: Characterization by cavity ringdown spectroscopy and quantum-chemical calculations

The cavity ringdown (CRD) absorption spectrum of 2-cyclohexen-1-one (2CHO) was recorded over the range 401.5-410.5 nm in a room-temperature gas cell. The very weak band system (ε ≤ 0.1 M(-1) cm(-1)) in this spectral region is due to the T1(n, π*) ← S0 electronic transition. The 0(0)(0) origin band was assigned to the feature observed at 24,558.8 ± 0.3 cm(-1). We have assigned 46 vibronic transitions in a region extending from -200 to +350 cm(-1) relative to the origin band. For the majority of these transitions, we have made corresponding assignments in the spectrum of the deuterated derivative 2CHO-2,6,6-d3. From the assignments, we determined fundamental frequencies for several vibrational modes in the T1(n, π*) excited state of 2CHO, including the lowest ring-twisting (99.6 cm(-1)) and ring-bending (262.2 cm(-1)) modes. These values compare to fundamentals of 122.2 cm(-1) and 251.9 cm(-1), respectively, determined previously for the isoconfigurational S1(n, π*) excited state of 2CHO and 99 cm(-1) and 248 cm(-1), respectively, for the S0 ground state. With the aid of quantum-mechanical calculations, we have also ascertained descriptions for these two modes, thereby resolving ambiguities appearing in the previous literature. The ring-twisting mode (ν39) contains a significant contribution from O=C-C=C torsion, whereas the ring-bending mode (ν38 in the ground state) involves mainly the motion of C-5 with respect to the plane containing the other heavy atoms. The CRD spectroscopic data for the T1(n, π*) state have allowed us to benchmark several computational methods for treating excited states, including time-dependent density functional theory and an equation-of-motion coupled cluster method. In turn, the computational results provide an explanation for observed differences in the T1(n, π*) vs. S1(n, π*) ring frequencies.