Theoretical spectroscopic parameters for isotopic variants of HCO+ and HOC.

Theoretical spectroscopic parameters are derived for all isotopologues of HCO+ and HOC+ involving H, D, 16O, 17O, 18O, 12C, and 13C by means of a two-step procedure. Full-dimensional rovibrational calculations are first carried out to obtain numerically exact rovibrational energies for J = 0-15 in both parities. Effective spectroscopic constants for the vibrational ground state, ν1, ν2, and ν3 are determined by fitting the calculated rovibrational energies to appropriate spectroscopic Hamiltonians. Combining our vibration-rotation corrections with the available experimental ground-state rotational constants, we also derive the new estimate for the equilibrium structure of HCO+, re(CH) = 1.091 98 Å and re(CO) = 1.105 62 Å, and for the equilibrium structure of HOC+, re(HO) = 0.990 48 Å and re(CO) = 1.154 47 Å. Regarding the spectroscopic parameters, our estimates are in excellent agreement with available experimental results for the isotopic variants of both HCO+ and HOC+: the agreement for the rotational constants Bv is within 3 MHz, for the quartic centrifugal distortion constants Dv within 1 kHz, and for the effective ℓ-doubling constants qv within 2 MHz. We thus expect that our results can provide useful assistance in analyzing expected observations of the rare isotopologues of HCO+ and HOC+ that are not yet experimentally known.

The B11244 story: rovibrational calculations for C3H(+) and C3H(-) revisited.

New theoretical values for the rovibrational parameters of C3H(+) and C3H(-) in the ground vibrational states are reported for the quartic internal coordinate force fields constructed by Huang et al. [Astrophys. J., Lett. 768, L25 (2013)] and Fortenberry et al. [Astrophys. J. 772, 39 (2014)]. Effective spectroscopic parameters are derived from the rovibrational energies, calculated up to J = 20 for C3H(+) and J = 12 for C3H(-) by means of a computational strategy for numerically exact rovibrational computations. Our results help to resolve a disharmony between the experimental observations and previous theoretical predictions in the case of C3H(+): we show that the previously used perturbational approach is not sufficient to reliably predict relevant spectroscopic properties of C3H(+) and that the force field of Huang et al. in combination with the numerically exact rovibrational treatment in fact supports the experimental identification of C3H(+), in contrast to the original conclusion of Huang et al. and Fortenberry et al. that the astronomical assignment of the B11244 carrier to C3H(+) is incorrect.

Formation of coherent rotational wavepackets in small molecule-helium clusters using impulsive alignment.

We show that rotational line spectra of molecular clusters with near zero permanent dipole moments can be observed using impulsive alignment. Aligned rotational wavepackets were generated by non-resonant interaction with intense femtosecond laser pump pulses and then probed using Coulomb explosion by a second, time-delayed femtosecond laser pulse. By means of a Fourier transform a rich spectrum of rotational eigenstates was derived. For the smallest cluster, C(2)H(2)-He, we were able to establish essentially all rotational eigenstates up to the dissociation threshold on the basis of theoretical level predictions. The C(2)H(2)-He complex is found to exhibit distinct features of large amplitude motion and very early onset of free internal rotor energy level structure.

Probing the structure and dynamics of molecular clusters using rotational wave packets.

Rotational wave packets of the weakly bound C(2)H(2)-He complex have been created using impulsive alignment. The coherent rotational dynamics were monitored for 600 ps enabling extraction of a frequency spectrum showing multiple rotational energy levels up to J = 4. spectrum has been combined with ab initio calculations to show that the complex has a highly delocalized structure and is bound only by ca. 7 cm(-1). The experiments demonstrate how highly featured rotational spectra can be obtained from an extremely cold environment where only the lowest rotational energy states are initially populated.

Rovibrational energies of the hydrocarboxyl radical from a RCCSD(T) study.

A RCCSD(T)/cc-pVQZ potential energy surface is constructed for the HOCO radical in the ground electronic state and used to compute rotation-vibration levels of HOCO and DOCO. Two numerical strategies are employed to study in detail the wave function properties. The importance of stretch-bend coupling, such as ν4/ν5 and ν3/ν4, for the internal dynamics is demonstrated. The rotational constants computed for the vibrational ground state of trans and cis conformers are in good agreement with experimental values.

Vibrational calculation for the HOCO radical and the cis-HOCO anion.

We present numerically exact vibrational transitions for trans-HOCO, cis-HOCO, and cis-HOCO(-) for the quartic force fields of Fortenberry et al. [J. Chem. Phys. 135, 134301 (2011); ibid. 135, 214303 (2011)], obtained by means of a computational strategy based on the discrete variable representation. Several adiabatic projection schemes have been employed to characterize the vibrational levels and to study the relevance of the intermode coupling (vibrational mixing). Our results help to clear up a large discrepancy between previously reported vibrational perturbation theory and vibrational configuration interaction predictions for the torsional frequency.

Isofulminic acid, HONC: Ab initio theory and microwave spectroscopy.

Isofulminic acid, HONC, the most energetic stable isomer of isocyanic acid HNCO, higher in energy by 84 kcal/mol, has been detected spectroscopically by rotational spectroscopy supported by coupled cluster electronic structure calculations. The fundamental rotational transitions of the normal, carbon-13, oxygen-18, and deuterium isotopic species have been detected in the centimeter band in a molecular beam by Fourier transform microwave spectroscopy, and rotational constants and nitrogen and deuterium quadrupole coupling constants have been derived. The measured constants agree well with those predicted by ab initio calculations. A number of other electronic and spectroscopic parameters of isofulminic acid, including the dipole moment, vibrational frequencies, infrared intensities, and centrifugal distortion constants have been calculated at a high level of theory. Isofulminic acid is a good candidate for astronomical detection with radio telescopes because it is highly polar and its more stable isomers (HNCO, HOCN, and HCNO) have all been identified in space.

Electric and magnetic properties of the four most stable CHNO isomers from ab initio CCSD(T) studies.

Electric and magnetic properties obtained from CCSD(T)/(aug-)cc-pCVXZ (X = T, Q, or 5) electronic structure calculations are reported for isocyanic acid, HNCO, cyanic acid, HOCN, fulminic acid, HCNO, and isofulminic acid, HONC, in their ground electronic states. Comparison of the theoretical results with the available experimentally derived values shows very satisfactory agreement. The new data should be helpful for the identification of these molecules due to characteristic hyperfine structure patterns in their microwave spectra. A brief discussion of the electronic structure properties, based on the electric field gradients, Mulliken population analysis of the total electron density, and molecular orbitals, is provided for the four CHNO isomers and the related HCN/HNC system.

Quasilinearity in tetratomic molecules: an ab initio study of the CHNO family.

We present coupled-cluster CCSD(T) all electron results for the equilibrium structure of isofulminic acid, HONC, together with results for the barrier to linearity and the energetics for the four most stable members of the CHNO isomer family, obtained for the ground electronic states by means of large correlation consistent basis sets. Minimum energy paths along the angular coordinates reported for these CHNO isomers are combined with the dominant kinetic energy contributions to predict key rovibrational spectroscopic features which are clearly reminiscent of quasilinear behavior in tetratomic molecules.

Six-dimensional potential energy surface and rovibrational energies of the HCCN radical in the ground electronic state.

We report large-scale quantum mechanical calculations for the HCCN radical in its ground electronic state. A six-dimensional potential energy surface based on MR-ACPF/cc-pVQZ ab initio energy points is developed and adjusted to reproduce experimental findings for and nu1 of HCCN. Rovibrational energy levels of HCCN and DCCN are computed for total rotational angular momentum J = 0-4 by making use of combined (functional + point wise) coordinate representations together with contraction schemes resulting from several diagonalization/truncation steps. The classical barrier to linearity is determined to be 287 cm(-1). Spectroscopic parameters are calculated for low lying states and compared with available experimental data. Energy patterns attributed to the nu4 bending mode and to the quasilinear nu5 bending mode are identified. It has been also found that nu2 and nu3 + (nu4(1),nu5(1))(0,0) are coupled in HCCN, while the mixing between nu3 and (2nu4(0), 2nu5(0))(0,0) is seen in DCCN.

Intermolecular interaction in an open-shell pi-bound cationic complex: IR spectrum and coupled cluster calculations for C2H2+-Ar.

The intermolecular potential energy surface (PES) of Ar interacting with the acetylene cation in its (2)Pi(u) ground electronic state is characterized by infrared photodissociation (IRPD) spectroscopy and quantum chemical calculations. In agreement with the theoretical predictions, the rovibrational analysis of the IRPD spectrum of C(2)H(2) (+)-Ar recorded in the vicinity of the antisymmetric CH stretching fundamental (nu(3)) is consistent with a vibrationally averaged T-shaped structure and a ground-state center-of-mass separation of R(c.m.) = 2.86 +/- 0.09 A. The nu(3) band experiences a blueshift of 16.7 cm(-1) upon complexation, indicating that vibrational excitation slightly reduces the interaction strength. The two-dimensional intermolecular PES of C(2)H(2) (+)-Ar, obtained from coupled cluster calculations with a large basis set, features strong angular-radial coupling and supports in addition to a global pi-bound minimum also two shallow side wells with linear H-bound geometries. Bound state rovibrational energy level calculations are carried out for rotational angular momentum J = 0-10 (both parities) employing a discrete variable representation-distributed Gaussian basis method. Effective spectroscopic constants are determined for the vibrational ground state by fitting the calculated rotational energies to the standard Watson A-type Hamiltonian for a slightly asymmetric prolate top.

Discrete variable approaches to tetratomic molecules: part I: DVR(6) and DVR(3) + DGB methods.

We present two discrete variable representation (DVR) based methods for the determination of the vibrational energy levels of tetratomic molecules. Both methods are designed for orthogonal internal coordinates in a body-fixed reference frame and make use of the DVR of three angular variables. The angular DVRs allow the construction of a fixed-angle three-mode Hamiltonian for the stretching vibrations. For each of the angular triples, the stretching eigenvalue problems are solved by employing 3D radial DVRs in the DVR(6) approach and real three-dimensional distributed Gaussian functions in the DVR(3) + DGB method. The angular degrees of freedom are taken sequentially into account in conjunction with a contraction scheme resulting from several diagonalization/truncation steps. Vibrationally averaged geometries, expectation values of rotational constants, and several adiabatic projection schemes developed in this work for tetratomic molecules are used to characterize the vibrational levels calculated by the DVR(6) and DVR(3) + DGB.

Discrete variable approaches to tetratomic molecules: part II: application to H2O2 and H2CO.

We have carried out large-scale calculations for accurate vibrational energy levels of formaldehyde and hydrogen peroxide. The discrete variable representations of the radial and angular coordinates are employed together with the contraction scheme resulting from several diagonalization/truncation steps. The global potential energy surface due to Carter et al. [J. Mol. Spectrosc. 90 (1997) 729] is used for H2CO and due to Koput et al. [J. Phys. Chem. A 102 (1998) 6325] for H2O2. For both molecules, the calculated vibrational energy levels are characterized by combining vibrationally averaged geometries and expectation values of rotational constants with several adiabatic projection schemes for automatic quantum number assignments. The energy levels of H2CO involving the excited v2 and v3 vibrations appear as resonances beyond the zero-order picture consisting of uncoupled 3D stretching and 2D bending modes. The torsional energy levels of H2O2 are studied in great detail and different energy patterns occurring below and above the cis barrier are discussed. Our full dimensional calculations for H2O2 have shown that the OH triad levels, 2vOH, are symmetry adapted local mode states.