Experimental characterization of the weakly anisotropic CN X 2Σ+ + Ne potential from IR-UV double resonance studies of the CN-Ne complex

IR-UV double resonance spectroscopy has been used to characterize hindered internal rotor states (n(K) = 0(0), 1(1), and 1(0)) of the CN-Ne complex in its ground electronic state with various degrees of CN stretch (ν(CN)) excitation. Rotationally resolved infrared overtone spectra of the CN-Ne complex exhibit perturbations arising from Coriolis coupling between the closely spaced hindered rotor states (1(1) and 1(0)) with two quanta of CN stretch (ν(CN) = 2). A deperturbation analysis is used to obtain accurate rotational constants and associated average CN center-of-mass to Ne separation distances as well as the coupling strength. The energetic ordering and spacings of the hindered internal rotor states provide a direct reflection of the weakly anisotropic intermolecular potential between CN X (2)Σ(+) and Ne, with only an 8 cm(-1) barrier to CN internal rotation, from which radially averaged anisotropy parameters (V(10) and V(20)) are extracted that are consistent for ν(CN) = 0-3. Complementary ab initio calculation of the CN X (2)Σ(+) + Ne potential using MRCI+Q extrapolated to the complete one-electron basis set limit is compared with the experimentally derived anisotropy by optimizing the radial potential at each angle. Experiment and theory are in excellent accord, both indicating a bent minimum energy configuration and nearly free rotor behavior. Analogous experimental and theoretical studies of the CN-Ne complex upon electronic excitation to the CN B (2)Σ(+) state indicate a slightly more anisotropic potential with a linear CN-Ne minimum energy configuration. The results from these IR-UV double resonance studies are compared with prior electronic spectroscopy and theoretical studies of the CN-Ne system.     

Experimental characterization of the CN X 2Σ+ + Ar and H2 potentials via infrared-ultraviolet double resonance spectroscopy

The hindered internal rotor states (n(K) = 0(0), 1(1), and 1(0)) of the CN-Ar complex with two quanta of CN stretch (v(CN) = 2), along with its ground state (v(CN) = 0), have been characterized by IR-UV double resonance and UV spectroscopy. Analysis of rotationally structured bands enable n(K) assignments and reveal perturbations due to Coriolis coupling between two closely spaced hindered rotor states, n(K) = 1(1) and 1(0). A deperturbation analysis is carried out to derive accurate rotational constants and their associated CN center-of-mass to Ar bond lengths as well as the magnitude of the coupling. The energetic ordering and spacings of the CN-Ar hindered rotor states provide a direct experimental probe of the angular dependence of the CN X (2)Σ(+) + Ar potential and permit radially averaged anisotropy parameters (V(10) = 5.2 cm(-1) and V(20) = 3.2 cm(-1)) to be determined. This analysis indicates a relatively flat potential about a linear N≡C-Ar configuration with a barrier to CN internal rotation of only ~12 cm(-1). The angular potentials determined from experiment and ab initio theory are in good accord, although theory predicts a higher barrier to CN internal rotation. A similar approach yields the infrared spectrum of H(2)-CN in the CN overtone region, which exhibits a rotationally resolved Σ ← Σ parallel band that is consistent with theoretical predictions for ortho-H(2)-CN.     

Electronic spectra of protonated benzaldehyde clusters with Ar and N2: effect of ππ* excitation on the intermolecular potential

Electronic spectra of the S(1)←S(0) transition of dimers of protonated benzaldehyde (BZH(+)) with Ar and N(2) are recorded by resonance-enhanced photodissociation in a tandem mass spectrometer. The S(1) origins observed are shifted to higher frequency upon complexation with Ar (ΔS(1) = 300 cm(-1)) and N(2) (ΔS(1) = 628 cm(-1)). Ab initio calculations at the CC2/aug-cc-pVDZ level suggest an assignment to H-bonded dimers of L = Ar and N(2) binding to the cis isomer of O-protonated BZH(+), yielding values of ΔS(1) = 242 and 588 cm(-1) for cis-BZH(+)-L(H). Electronic ππ* excitation results in a substantial increase of the proton affinity of BZH(+), which in turn destabilizes the intermolecular H-bonds to the inert ligands by 35%. The drastic effects of electronic ππ* excitation on the geometric and electronic structure as well as the strength and anisotropy of the intermolecular potential (H-bonding and π-bonding) are investigated.     

The effect of collisions on the intensity and polarization of laser-induced D1π → X1Σ+ + fluorescence from NaK

Laser-induced fluorescence was applied for determining total deactivation cross sections of the level (17,94) D¹Π of NaK molecules in collisions with He, Ne, Ar, Kr, Xe, as well as with K atoms. Relaxation cross sections of population and alignment were found to coincide within experimental error. The kinetics of the transient process was used for determining the cross section of thermalization of the optically depopulated level of the electronic ground state (5,67) X¹Σ⁺ in NaK + Ar collisions.     

Perturbations in the CN(B 2Σ+−X 2Σ+) tail band system. Vibrational assignment of the B 2Σ+ ∼ 4Π perturbation

The quantum numbers of the vibrational levels for the ⁴Π state which perturb the ν_B = 14 and 17 levels of the B ²Σ⁺ state are assigned by an analysis of the vibrational dependence of the B ²Σ⁺ ∼ ⁴Π perturbation constants. The vibrational levels involved in the B ²Σ⁺ ∼ ⁴Π perturbation, (ν_B, ν_Π) = (9, 1), (12, 4), (14, 7), (17, 12), and (18, 14), are consistent with the results of our recent ab initio calculation.     

Decay of the A2Σ+ and B2II quasibound states of HeH

The X²Σ⁺, A²Σ⁺ and B²II wavefunctions of the HeH molecule are calculated as single configurations state functions built from molecular orbitals generated by the Fock operator of the HeH⁺ ion. Dipole transition moments and non-adiabatic coupling matrix elements are evaluated in order to study the competition between dissociative photoemission and predissociation of the A and B states. Particular attention is paid to the X-A (2σ–3σ) avoided crossing and comparisons are made between its characteristics and those of the corresponding 2¹Σ⁺–3¹Σ⁺ (2σ–3σ) avoided crossing in the HeH⁺ ion. The predissociation lifetime of the A state is found to be of the order of 10⁻¹³ s which precludes its decay via dissociative photoemission in agreement with the analysis of recent translational spectroscopy experiments. The predissociation lifetime of the B state is found to be of the same order of magnitude as its radiative lifetime (i.e. of the order of a few 10⁻⁸ s). It is still not understood why this state would radiate rather than predissociate as has been inferred from experiment.     

The potential energy curves and probability density distributions of the X1Σ+ and A1Σ+ states of RBH

The potential energy curves PMO—RKR—van der Waals of the electronic A¹Σ⁺ and X₁Σ⁺ states of RbH have been determined. The potentials obtained are self-consistent with the experimental data because they have been tested by direct numerical solution of the radial Schrödinger equation. From exact vibrational eigenfunctions probability density distributions and Franck—Condon factors have been calculated over the range of vibrational levels observed. It is observed that the anomalous behaviour of the A¹Σ⁺ state arises in the υ′ = 1, 2 and 3 levels with probability density functions similar to those of a harmonic oscillator.     

Electric dipole moments of the MgO B1Σ+ and X1Σ+ states

The electric dipole moment of the excited B¹∑⁺ (υ′ = 0) state of MgO produced in a gas-phase reaction of Mg(¹S) atoms and N₂O is measured using the technique of Stark quantum-beat spectroscopy. It is shown to be |μ_υ′=0| = 5.94(24) D. The lifetime of the excited B¹∑⁺ (υ′ = 0) state is determined as τ = 22.5(1.5) ns. Using laser excitation spectroscopy and by direct observation of the P(1) line splitting at high electric field strengths in the B¹∑⁺−X¹∑⁺ (0,0) system the ground state electric dipole moment is measured yielding |μ_υ=0| = 6.2(6) D. Furthermore an electric-field-induced Q branch is observed.     

Rotational analysis and deperturbation of the A2Π → X2Σ+ and B'2Σ+ → X2Σ+ emission spectra of MgH

Deperturbation analysis of the A(2)Π → X(2)Σ(+) and B(')(2)Σ(+) → X(2)Σ(+) emission spectra of (24)MgH is reported. Spectroscopic data for the v = 0 to 3 levels of the A (2)Π state and the v = 0 to 4 levels of the B'(2)Σ(+) state were fitted together using a single Hamiltonian matrix that includes (2)Π and (2)Σ(+) matrix elements, as well as off-diagonal elements coupling several vibrational levels of the two states. A Dunham-type fit was performed and the resulting Y(l,0) and Y(l,1) coefficients were used to generate Rydberg-Klein-Rees (RKR) potential curves for the A (2)Π and the B'(2)Σ(+) states. Vibrational overlap integrals were computed from the RKR potentials, and the off-diagonal matrix elements coupling the electronic wavefunctions (a(+) and b) were determined. Zero point dissociation energies (D(0)) of the A(2)Π and B'(2)Σ(+) states of (24)MgH were determined to be 12,957.5 ± 0.5 and 10,133.6 ± 0.5 cm(-1), respectively. Using the Y(0,1) coefficients, the equilibrium internuclear distances (r(e)) of the A(2)Π and B'(2)Σ(+) states were determined to be 1.67827(1) ? and 2.59404(4) A?, respectively.     

Self-consistent potential energy curves for the A1Σ+ and X1Σ+ states of isotopic cesium hydrides

From the spectroscopic experimental data available in the literature we have determined the mass-reduced Dunham coefficients for the A¹Σ⁺ ↔ X¹Σ⁺ system of the isotopic species CsH and CsD. Based upon these results, for both ground and excited states of cesium hydride and deuteride we report new hybrid rotationless potential energy curves (PMO-RKR-van der Waals) up to the dissociation. As a consistency check on the accuracy of the potentials the eigenvalues were calculated by direct numerical integration of the radial Schrödinger equation and found to agree within the rms error 0.39 cm⁻¹ (X¹Σ⁺) and 0.41 cm⁻¹ (A¹Σ⁺) with the experimental vibrational energies. From the wavefunctions, the rotational constants B_υ, centrifugal distortion terms D_υ, H_υ and L_υ, Franck—Condon factors, and probability density distributions were obtained. The probability density distributions for the lowest vibrational levels of the A¹Σ⁺ show an anharmonicity associated with the anomalous behavior of that state.     

Vibronic coupling in the A2Π and B2Σ+ electronic states of the NCS radical

The spin-rovibronic energy levels of the A(2)Π and B(2)Σ(+) electronic states of thiocyanate radical have been calculated variationally, using high-level ab initio coupled diabatic potential energy surfaces. Computations up to J = 7∕2 have been performed, obtaining all levels with K ≤ 3 (Σ(1/2),Π(1/2,3/2),Δ(3/2,5/2),Φ(5/2,7/2)), for energies up to 2000 cm(-1) above the A(000)(2)Π(3∕2) level. The available experimental data have been critically reviewed in the light of the theoretical findings.     

Spectroscopic (fluorescence, 1D-ROESY) and theoretical studies of the thiabendazole and β-cyclodextrin inclusion complex

The inclusion complex between the anti-helminthic drug thiabendazole (TBZ) and the β-cyclodextrin (βCD) was characterized in solution using fluorescence and ¹H-Nuclear Magnetic Resonance spectroscopy and studied theoretically by semi empirical PM3 and density functional theory (DFT) quantum mechanical calculations. Thermodynamic stability associated with the formation of the TBZ:βCD inclusion complex in aqueous solution was determined treating the drug’s fluorescence enhancement in the presence of cyclodextrin by a non-linear model, which indicated a moderate host–guest affinity at equilibrium (K 150 ± 31 at 25 °C). Its supramolecular structure in solution was studied through the 1D-ROESY NMR experiment, which produced evidence that the guest molecular encapsulation occurs preferably via the drug’s benzimidazole group. Theoretical study employing molecular optimization with the semi empirical PM3 method provided two energetic-equivalent complex structures that are in accordance with the NMR experimental evidences. Single point energy calculations with DFT at the B3LYP/6-31G (d,p) level suggest the most stable structure of the inclusion complex and further comprehension on the interactions and conformational strains involved in its formation.     

A comparison between the potential energy curves for the X1Σg+ state of H2 and D2

The electronic potential for the ground state of H₂ and D₂, molecules has been calculated from spectroscopic molecular constants. Numerical integration of the radial wave equation gives accurate self-consistent values (an eigenvalue mean deviation of about 1 cm⁻¹). A comparison between different potentials is reported.     

Theoretical studies on the potential energy surfaces and vibrational energy levels of HXeF and HXeCl

The potential energy surfaces for the electronic ground state of the HXeCl and HXeF molecules areconstructed by using the internally contracted multi-reference configuration interaction with theDavidson correction(icMRCI Q)method and large basis sets.The stabilities and dissociation barriersare identified from the potential energy surfaces.The three-body dissociation channel is found to bethe dominate dissociation channel for HXeCl,while two dissociation channels are possible and com-petitive for HXeF.Based on the obtained potentials,vibrational energy levels of HXeCl and HXeF arecalculated using the Lanczos algorithm.Our theoretical results are in good agreement with the avail-able observed values.Particularly,the calculated fundamental frequency of the H—Xe stretching vi-bration including the Xe matrix effect of HXeCl is found to be 1666.6 cm-1,which is only 17.6 cm-1higher than the recently observed value of 1649 cm-1.     

Identification of the C2∏-X2Σ+ band system of AlO in the ultraviolet galvanoluminescence obtained during aluminum anodization

The first galvanoluminescence spectrum in the ultraviolet region obtained during anodization of high purity aluminum samples annealed at temperature above 525°C is presented. An intense broad peak with the maximum at about 31,900 cm(-1) is assigned to the transitions (some of them heretofore unobserved) between vibrational levels of the C(2)∏→X(2)Σ(+) spectral system of AlO, partly overlapped with the A(2)Σ(+)→X(2)∏ system of OH.     

A CAS SCF CI study of the 1Σ+g and 3IIu states of the C2 molecule and the 4Σ−g and 2IIu states of the C+2 ion

Ab initio calculations of the X ¹Σ⁺_g and a ³II_u states of C₂ and the X⁴Σ⁻_g and a²II_u states of the C⁻₂ molecular ion are performed to determine the corresponding potential curves around the potential minima and at the dissociation limits. A large Gaussian basis set augmented by three d-type polarization functions on each carbon center is used to approximate the molecular orbits. The calculations are done at the complete-active-space SCF and multi-reference configuration interactions level. Spectroscopic constants and rotation—vibration energies are derived from the ab initio calculated potentials. Good agreement between theory and experiment is obtained for the X¹Σ⁺_g and a ³II_u states of C₂. In the earlier tentative assignment of the observed electronic transition around 2490 Å to the ²Σ⁻_g ← ²II_u system in C⁺₂, the lower state is confirmed by the present calculations to be C⁺ ₂ (²II_u).     

Laser-induced fluorescence detection of N+2(X 2Σ+g) resulting from the He(23S) + N2 and He+, He+2 + N2 reactions in a flowing afterglow

The formation processes of N⁺₂ (X ²Σ⁺_g) resulting from the He(2 ³S) + N₂ Penning ionization and the thermal energy He⁺, He⁺₂ + N₂ charge transfer reaction are studied by observing the N⁺₂ (B ²Σ⁺_u ← X ²Σ⁺_g) laser-induced fluorescence (LIF) in a flowing afterglow. In both reactions, the vibrational population) decrease monotonically with increasing vibrational quantum number from υ″ = 0 to 3, and a population inversion with a peak at υ″ = 4 is seen. In the He(2 ³S) + N₂ Penning ionization, the vibrational populations of N⁺₂ (X, υ″ = 0–2) are explained by a direct channel and the B —X radiative cascade, while those of N⁺₂ (X, υ″ = 4–6) are ascribed to the collision-induced electronic energy transfer between the A ²Π_u and X ²Σ⁺_g states. In the He⁺, He⁺₂ + N₂ reaction, the N⁺₂ (X, υ″ = 0–3) is interpreted as the B−X radiative cascade and the collisional quenching of the unidentified states produced from the He⁺ + N₂ reaction, while the collision-induced electronic energy transfer from the N⁺₂ (A) state produced through the He⁺₂ + N₂ reaction is probably important for the formation of N⁺₂ (X, υ″ = 4–6).     

Comment on “ab initio investigation of low-lying 2Σ+ and 2Π states of NO2+”

Energies of the singlet stales of the doubly charged nitric oxide cation have been determined experimentally by the technique of double charge transfer, and are compared with previous results of full-valence MC SCF plus first-order CI techniques. The good agreement obtained permits an assignment of some of the peaks in the experimental spectrum, and confirms the accuracy of these calculations.     

Quasiclassical trajectory study of the postquenching dynamics of OH A 2Σ+ by H2/D2 on a global potential energy surface

We report full-dimensional, electronically adiabatic potential energy surfaces (PESs) for the ground state (1A(')) and excited state (2A(')) of OH(3). The PESs are permutationally invariant fits to roughly 23,000 electronic energies (MRCI + Q/aVTZ). Classical trajectory calculations of the postquenching dynamics of OH A (2)Σ(+) are carried out on the 1A(') PES for H(2) and D(2), at previously identified conical intersections (CoIs) [B. C. Hoffman and D. R. Yarkony, J. Chem. Phys. 113, 10091 (2000)]. The initial momenta are sampled fully and partially microcanonically, corresponding to "adiabatic" and "diabatic" models of the dynamics, respectively. Branching ratios of reactive to nonreactive channels from separate C(2v), C(∞v), and C(s) symmetries of CoIs are calculated, as are final rovibrational state distributions of OH and H(2) products. The rovibrational distributions of the OH and D(2) products, the D/H-atom translational energy distribution are calculated and compared to experimental ones. Agreement for these observable quantities is good. The branching between reactive and nonreactive quenching is sensitive to the momenta sampling; very good agreement with experiment is obtained using the diabatic sampling but not with the adiabatic sampling. The vibrational state distributions of H(2)O and HOD (although not measured by experiment) are also presented.