Three-Parameter Electric Dipole Moment Function for the CO Molecule.

A global electric dipole moment function of the ground electronic state of carbon monoxide is constructed by morphing its best theoretical approximants from the literature to the best available experimental data within the framework of the reduced radial curve approach. The resulting functions coincide with their best many-parameter empirical counterparts so closely that they can be used as highly accurate three-parameter representations. Apparently, given the mathematical nature of the problem addressed, this approach can be applied equally well to all radial molecular functions that have similarly cumbersome shapes as the function probed. This means that the property characteristics of diatomic molecules can, in principle, be described with high precision even when as few as three pertinent experimental data points are accurately known. To date, no such functional approximants are available in the literature.

Reduced Radial Curves of Diatomic Molecules.

The prospect of using the concept of a universal reduced potential energy curve (RPC) for a broader class of radial molecular functions is explored by performing appropriate model calculations for the electric dipole moment functions of the hydrogen halides HF, HCl, and HBr. The reduced radial functions of the model systems, constructed from their best available theoretical approximants, coincide so closely that they can be used as few-parameter universal representations of functions available in the literature. Given the mathematical nature of the problem addressed here, the results are not limited to the functions studied but can be applied equally well to all radial molecular functions that have similar shapes, such as electric quadrupole moment and dipole polarizability functions.

Hydrogen Bonding with Hydridic Hydrogen-Experimental Low-Temperature IR and Computational Study: Is a Revised Definition of Hydrogen Bonding Appropriate?

Spectroscopic characteristics of Me3Si-H···Y complexes (Y = ICF3, BrCN, and HCN) containing a hydridic hydrogen were determined experimentally by low-temperature IR experiments based on the direct spectral measurement of supersonically expanded intermediates on a cold substrate or by the technique of argon-matrix isolation as well as computationally at harmonic and one-dimensional anharmonic levels. The computations were based on DFT-D, MP2, MP2-F12, and CCSD(T)-F12 levels using various extended AO basis sets. The formation of all complexes related to the redshift of the Si-H stretching frequency upon complex formation was accompanied by an increase in its intensity. Similar results were obtained for another 10 electron acceptors of different types, positive σ-, π-, and p-holes and cations. The formation of HBe-H···Y complexes, studied only computationally and again containing a hydridic hydrogen, was characterized by the blueshift of the Be-H stretching frequency upon complexation accompanied by an increase in its intensity. The spectral shifts and stabilization energies obtained for all presently studied hydridic H-bonded complexes were comparable to those in protonic H-bonded complexes, which has prompted us to propose a modification of the existing IUPAC definition of H-bonding that covers, besides the classical protonic form, the non-classical hydridic and dihydrogen forms.

The stability of covalent dative bond significantly increases with increasing solvent polarity.

It is generally expected that a solvent has only marginal effect on the stability of a covalent bond. In this work, we present a combined computational and experimental study showing a surprising stabilization of the covalent/dative bond in Me3NBH3 complex with increasing solvent polarity. The results show that for a given complex, its stability correlates with the strength of the bond. Notably, the trends in calculated changes of binding (free) energies, observed with increasing solvent polarity, match the differences in the solvation energies (ΔEsolv) of the complex and isolated fragments. Furthermore, the studies performed on the set of the dative complexes, with different atoms involved in the bond, show a linear correlation between the changes of binding free energies and ΔEsolv. The observed data indicate that the ionic part of the combined ionic-covalent character of the bond is responsible for the stabilizing effects of solvents.

Dissociative recombination of N2H+ ions with electrons in the temperature range of 80-350 K.

Recombination of N2H+ ions with electrons was studied using a stationary afterglow with a cavity ring-down spectrometer. We probed in situ the time evolutions of number densities of different rotational and vibrational states of recombining N2H+ ions and determined the thermal recombination rate coefficients for N2H+ in the temperature range of 80-350 K. The newly calculated vibrational transition moments of N2H+ are used to explain the different values of recombination rate coefficients obtained in some of the previous studies. No statistically significant dependence of the measured recombination rate coefficient on the buffer gas number density was observed.

Localised quantum states of atomic and molecular particles physisorbed on carbon-based nanoparticles.

The vibrational states of atomic and molecular particles adsorbed on long linear nanographenes are described using reliable theoretical potentials and appropriate vibrational (lateral) Hamiltonians. Although they rigorously obey the Bloch theorem only for infinite nanographenes, the energy patterns of the probed states closely resemble the usual Bloch bands and gaps. In addition, for any finite nanographene, these patterns are enriched by the presence of "solitary" energy levels and the "resonance" structure of the bands. While typical band states are profoundly delocalised due to a fast tunneling of the adsorbed particle, the "solitary" and "resonance" states exhibit strong localisation, similar to the behaviour of the states of the Wannier-Stark ladders in optical and semiconductor superlattices.

Highly Sensitive Ammonia Probes of a Variable Proton-to-Electron Mass Ratio.

The mass sensitivity of the vibration-rotation-inversion energy levels of ammonia is probed using the nonrigid inverter theory. It is shown that the sensitivity exhibits non-negligible centrifugal distortion dependence, which is currently disregarded. The centrifugal distortion effects are especially important in the case of the Δk = ±3 "forbidden" transitions involving accidentally coinciding ro-inversional states |a,J,K = 3⟩ and |s,J,K = 0⟩ of the ν2 vibrational state of (14)NH3. The energy differences of these states exhibit very anomalous mass sensitivities (see Jansen etal. J. Chem. Phys. 2014 , 140 , 010901 ), thus appearing as new highly sensitive probes of the cosmological variability of the proton-to-electron mass ratio.

Effective potential energy curves of the ground electronic state of CH+.

This study presents effective (mass-dependent) potential energy curves for the methylidyne cation, which reproduce highly accurately all the available spectral data and allow for evaluation of reliable ro-vibrational wavefunctions of the probed isotopomers. The ro-vibrational wavefunctions are then used to average ab initio calculated radial functions of the rotational g-factor and spin-rotation constants yielding rotational and vibrational matrix elements of these properties for specific ro-vibrational states or transition moments for all isotopomers. The results can be of use in answering open questions concerning the formation/destruction of CH(+) in the interstellar medium and in the assignment of Zeeman or hyperfine splittings in rotational spectra of CH(+).

Vibrational energies of LiH2(+) and LiD2(+) in the Ã1Σ+ electronic state.

In connection with the recent study of the ground electronic state of the LiH2(+) molecular ion (Kraemer, W. P.; Spirko, V. Chem. Phys. 2006, 330, 190), the adiabatic three-dimensional double-minimum potential enery surface of the first excited electronic state was evaluated, including its two lowest atom-diatom dissociation channels as well as the three-atom complete fragmentation asymptote. Applying the Sutcliffe-Tennyson Hamiltonian for triatomic molecules, the levels of all bound vibrational states and the levels of the states localized in the two energy minimum regions were separately determined. The validity of statistical methods such as the density of states approach and the nearest-neighbor level spacing distribution (NNSD) was tested for the light LiH2(+) ion. Special effort was put into investigating possible effects of a tunnelling motion across the proton-transfer barrier on the vibrational level pattern using the NNSD approach.

Ab initio vibrational dynamics of molecular hydrogen on graphene: An effective interaction potential.

The interaction potential confining the stretching and translational motions of a molecular hydrogen physisorbed on the graphene surface has been calculated by means of the DFT/CC approach. Using a simple adiabatic separation of the stretching and translational motions, a set of effective stretching potentials is generated by performing a "finite box" integrating over the translational degrees of freedom. The resulting potentials, forming energetically narrow bands, are used to evaluate the corresponding average stretching energies, which are in turn compared to their experimental counterparts. The mass-dependent "translational" corrections of the purely stretching potential significantly improve the theory versus experiment agreement, thus evidencing their importance in the physisorption processes. Although not fully quantitative, the DFT/CC stretching potentials seem to exhibit physically correct shapes, as their morphing by only a few parameters allows for a quantitative fitting of the observed vibrational energies in terms of the effective (mass-dependent) interaction potentials.

On the elusive twelfth vibrational state of beryllium dimer.

The beryllium dimer has puzzled chemists for roughly 80 years on account of its unexpectedly strong bonding interaction between two nominally closed-shell atoms. Recent spectroscopic measurements characterized the molecule's ground electronic state with sufficient resolution to distinguish 11 vibrational levels; the possibility that a twelfth level lay just below the dissociation threshold remained unresolved. Here we present a potential function, based on ab initio calculations at the full configuration interaction level, that definitively supports the presence of this twelfth vibrational state. "Morphed" versions of this potential, fitted to experimental data, closely reproduce the observed spectra to within 0.1 cm(-1), bolstering the strength of the assignment.

The structure and vibrational dynamics of the pyrrole dimer.

The energy, dynamical geometry characteristics and low frequency intermolecular vibrations of the pyrrole dimer have been examined at the MP2 and CCSD(T) levels of ab initio theory. The actual distortions of the pyrrole dimer from its reference (equilibrium) position were measured using the distance of the monomer mass centres (R), the angle between their planes (the mirror planes orthogonal to the molecular planes of both monomers were assumed to coincide) and the angle between the R directional vector and the proton-accepting monomer plane; the structures of the monomers were assumed to be unchanged by dimerisation. The lowest part of the potential energy function confining the probed motions possessed two equivalent energy pockets with the CCSD(T)/complete basis set limit stabilisation energy of 6.2 kcal mol(-1) separated by a relatively low barrier (0.8 kcal mol(-1)), thus raising questions concerning the classical interconversion of the T-shaped equilibrium structures via a C(2h) parallel-displaced transient structure and/or quantum mechanical tunnellings through the barrier. The questions have been answered unequivocally by calculating the energies and tunnelling splittings of the relevant vibrational levels. Importantly: (a) all the excited tunnelling (interconverting) states underwent fast geometry interconversions, hence evidencing conformational instability of the studied dimer under usual laboratory conditions; (b) the dynamical averages of the used geometry characteristics exhibited profound tunnelling (interconverting) dependences, thus advocating that they be respected in reliable structural studies of the pyrrole dimer and chemically similar systems; (c) the geometry characteristics of the ground vibrational state agreed quite reasonably with their experimental counterparts, evidencing the adequacy of the theory used and the reliability of the characteristics predicted for the excited vibrational states; and (d) the calculated dissociation barrier of the dimer exceeds its experimentally derived analogue by more than three times, showing the inadequacy of the constraining assumptions used to derive it from the experimental spectra.

The ab initio assigning of the vibrational probing modes of tryptophan: linear shifting of approximate anharmonic frequencies vs. multiplicative scaling of harmonic frequencies.

To gain insight into the prospects for a few-dimensional ab initio quantum-mechanical description of the vibrational motions of conformationally flexible molecular systems, the NH-, NH(2)-, CO- and OH-stretching and COH-bending vibrations of the most stable tryptophan conformations have been probed using simple one- and two-dimensional anharmonic Hamiltonians and potential energy functions evaluated by means of the standard RI-MP2, CCSD(T) and DFT-D quantum chemical procedures. Although strongly dependent on the procedure used, the calculated vibrational spectral patterns have been found to be in a robust one-to-one harmony with their experimental counterparts, thus proving the adequacy of the theory used for the reliable assignment of the experimental data. Therefore, the approach appears to be a suitable tool for assigning the vibrational probing modes even of systems which are too large to be tractable by the standard normal-coordinate analysis.

Probing the flexibility of internal rotation in silylated phenols with the NMR scalar spin-spin coupling constants.

The rotation of a trimethylsiloxy (TMSO) group in three silylated phenols (with three different ortho substituents -H, -CH3, and -C(CH3)3) was studied with the NMR (n)J(Si,C), n = 2, 3, 4, 5, scalar spin-spin coupling between the (29)Si nucleus of the TMSO group and the (13)C nuclei of the phenyl ring. The internal rotation potential calculated with the B3LYP and MP2 calculation methods including the effect of a solvent environment (gas phase, chloroform, and water) was used for the calculation of the dynamical averages of the scalar coupling constants in the framework of the rigid-bender formalism. Solvent effects, the quality of the rotational potential, and the applicability of the classical molecular dynamic to the problem is discussed. Quantum effects have a sizable impact on scalar couplings, particularly for the internal rotational states well localized within the wells of the potential surfaces for the TMSO group. The overall difference between the experimental and theoretical scalar couplings calculated for the global energy-minima structures (static model) decreases substantially for both model potentials (B3LYP, MP2) when the molecular motion of the TMSO group is taken into account. The calculated data indicate that the inclusion of molecular motion is necessary for the accurate calculation of the scalar coupling constants and their reliable structural interpretation for any system which possesses a large-amplitude motion.

Assigning the NH stretches of the guanine tautomers using adiabatic separation: CCSD(T) benchmark calculations.

Using an adiabatic separation of the NH stretching vibration from the remaining vibrational molecular motions, the NH fundamental frequencies and absolute intensities of several keto/enol and 7/9NH tautomers of guanine are evaluated ab initio within the framework of a one-dimensional "semirigid" stretching Hamiltonian. The frequencies (calculated by means of the standard MP2, CCSD(T) and DFT procedures) are in a close one-to-one harmony with their experimental counterparts, thus evidencing the adequacy of the used separation for reliable assigning of the NH stretches in the vibrational spectra of very large molecular systems.

Dependence of the L-alanyl-L-alanine conformation on molecular charge determined from ab initio computations and NMR spectra.

The l-alanyl-l-alanine (AA) molecule behaves differently in acidic, neutral, and basic environments. Because of its molecular flexibility and strong interaction with the aqueous environment, its behavior has to be deduced from the NMR spectra indirectly, using statistical methods and comparison with ab initio predictions of geometric and spectral parameters. In this study, chemical shifts and indirect spin-spin coupling constants of the AA cation, anion, and zwitterion were measured and compared to values obtained by density functional computations for various conformers of the dipeptide. The accuracy and sensitivity of the quantum methods to the molecular charge was also tested on the (mono)-alanine molecule. Probable AA conformers could be identified at two-dimensional potential energy surfaces and verified by the comparison of the computed parameters with measured NMR data. The results indicate that, whereas the main-chain peptide conformations of the cationic (AA+) and zwitterionic (AAZW) forms are similar, the anion (AA-) adopts also another, approximately equally populated conformer in the aqueous solution. Additionally, the NH2 group can rotate in the two main chain conformations of the anionic form AA-. According to a vibrational quantum analysis of the two-dimensional energy surfaces, higher-energy conformers might exist for all three charged AA forms but cannot be detected directly by NMR spectroscopy because of their small populations and short lifetimes. In accord with previous studies, the NMR parameters, particularly the indirect nuclear spin-spin coupling constants, often provided an excellent probe of a local conformation. Generalization to peptides and proteins, however, has to take into account the environment, molecular charge, and flexibility of the peptide chain.

Quasiplanarity of the peptide bond.

The vibrational motions of the model peptide unit represented by the main-chain carbonyl carbon, oxygen, nitrogen, and amide hydrogen are analyzed quantum-mechanically using formamide, cis-N-methylformamide, trans-N-methylformamide, N,N-dimethylformamide, l-alanyl-l-alanine, and N-benzoylphenylalanine as dynamical models. To make this analysis computationally feasible, the peptide unit vibrational motions were first separated from the remaining molecular vibrational motions by means of the crude adiabatic (Born-Oppenheimer) approximation, and then, using the same approximate separation, the peptide unit dynamical problem was separated into sets of high- and low-frequency subproblems. Importantly, the simplest dynamical (one-dimensional) problem based on the separation of the amide out-of-plane motion from the rest of the peptide unit motions allows for a physically correct description of the effective "ground state" molecular geometry of all studied systems. The separation is thus believed to be also suitable for reliable estimation of the dynamical effects on the geometry of the peptide unit in other molecular systems.

Tuning ab initio data to scattering length: the a (3)Sigma(+) state of KRb.

Interaction energies for the lowest triplet state a (3)Sigma(+) of KRb are calculated using high level ab initio methods. The interaction energies are then morphed so that the resulting potential energy curve yields 32 bound states and the correct scattering length for (40)K(87)Rb. Calculated vibrational spacings are shown to be in very good agreement with the available experimental Fourier transform and photoassociation vibrational data, but a different numbering scheme has to be used for the experimental vibrational assignment.

Theoretical study of photoacidity of HCN: the effect of complexation with water.

The character of the hydrogen bonding and the excited state proton transfer (ESPT) in the model system HCN...H(2)O is investigated. The PES of the two lowest excited states of the H(2)O...HCN complex was calculated using the CASPT2 method. The nonadiabatic coupling of the two states of the (pi-->pi*) and (pi-->sigma*) character is responsible for the excited state proton/hydrogen transfer. Compared to the ground state, the barrier for this process is significantly smaller. An increased number of water molecules in the complex with cyclic hydrogen-bonded network causes a large blue shift of the state of the (pi-->sigma*) character. The question of the dissociation of the complex in its excited state is also addressed.

Vibrational predissociation of H5 +.

The full nine-dimensional vibrational Hamiltonian for H5 + described in the literature [Kraemer et al., J. Mol. Spectrosc. 164, 500 (1994)] is adopted here for an approximate evaluation of the spectral linewidths of the observed H-H stretching modes of the H5 + ion and the corresponding modes of its D5 + isotopomer. In this approximation the high dimensionality of the original Hamiltonian is reduced to a three-dimensional model Hamiltonian which takes only the H-H stretching modes and the molecular dissociation mode into consideration assuming that they are adiabatically separable from the remaining modes. To make the calculations numerically feasible, the molecular degenerate ("skeletal") vibrations are assumed to take place in harmonic potentials, and the effect of the internal propeller rotation is completely disregarded. The linewidths calculated in this approximation are too small to explain the broad shapes of the observed spectral transitions. It can thus be argued that the failure to resolve rotational structure in the observed bands is mainly due to spectral congestion and only partly due to predissociation of the H5 + cluster.