HNC Isomerization Research Articles

Computed spectroscopic properties of HCN, HNC, and all their D, 13C, and 15N substituted isotopologues

ABSTRACT Starting from ab initio electronic structure data, we develop parametrized analytic potential energy surfaces for the HCN and HNC isomers by variationally calculating rovibrational energy levels and adjusting the potential parameters so as to get agreement with experimentally derived transition frequencies to within about 1 cm−1. We also determine an analytic expression in terms of molecular parameters to effortlessly calculate the rovibrational energy levels. We use the obtained empirical potentials to calculate rovibrational levels for eight isotopologues of HCN and eight of HNC up to about 4000 cm−1 above the ground state. The energy levels are estimated to be accurate to within about 3 cm−1 based on comparison to experimental rovibrational transition frequencies for H12C14N, H12C14N, H13C14N, and H12C15N. For all 16 isotopologues, we calculate the zero-point energy and in nine cases we can compare with experimentally derived values. In these comparisons, the variationally obtained ZPE is within 5 cm−1 of the experimentally derived value, while the closed expression gives values within 6 cm−1 of the experimental values. For all 16 isotopologues, we also give molecular parameters from which the energy levels can easily be calculated using the closed expression. Endo- and exoergicities are given for 12 isotopic exchange reactions involving HCN/HNC and some isotopologues together with pre-exponential factors that should be useful in future modelling studies of rare isotopologues.

Cyanides, Isocyanides, and Hydrides of Zn, Cd and Hg from Metal Atom and HCN Reactions: Matrix Infrared Spectra and Electronic Structure Calculations.

Zinc and cadmium atoms from laser ablation of the metals and mercury atoms ablated from a dental amalgam target react with HCN in excess argon during deposition at 5 K to form the MCN and MNC molecules and CN radicals. UV irradiation decreases the higher energy ZnNC isomer in favor of the lower energy ZnCN product. Cadmium and mercury atoms produce analogous MCN primary molecules. Laser ablation of metals also produces plume radiation which initiates H‐atom detachment from HCN. The freed H atom can add to CN radical to produce the HNC isomer. The argon matrix also traps the higher energy but more intensely absorbing isocyanide molecules. Further reactions with H atoms generate HMCN and HMNC hydrides, which can be observed by virtue of their C−N stretches and intense M−H stretches. Computational modeling of IR spectra and relative energies guides the identification of reaction products by providing generally reliable frequency differences within the Zn, Cd and Hg family of products, and estimating isotopic shifts using to 13C and 15N isotopic substitution for comparison with experimental data.

Calculation of Transition State Energies in the HCN-HNC Isomerization with an Algebraic Model.

Recent works have shown that the spectroscopic access to highly excited states provides enough information to characterize transition states in isomerization reactions. Here, we show that information about the transition state of the bond-breaking HCN-HNC isomerization reaction can also be achieved with the two-dimensional limit of the algebraic vibron model. We describe the system's bending vibration with the algebraic Hamiltonian and use its classical limit to characterize the transition state. Using either the coherent state formalism or a recently proposed approach by Baraban [ Science 2015 , 350 , 1338 - 1342 ], we obtain an accurate description of the isomerization transition state. In addition, we show that the energy-level dynamics and the transition state wave function structure indicate that the spectrum in the vicinity of the isomerization saddle point can be understood in terms of the formalism for excited-state quantum phase transitions.

Manipulation of Goos–Hänchen shifts in a HCN→HNC isomerization with permanent dipole moments

The behavior of the Goos-Hanchen shifts of the reflected probe light beam at the interface of a three-layer cavity with a three-level Λ-type molecular system is theoretically investigated. The molecular system is HCN→HNC isomerization with nonzero permanent dipole moments (PDMs). The effect of electromagnetically induced transparency (EIT) induced by PDMs in molecules is studied for (1+1)- and (1+2)-transition processes. Thus, the optical response of the medium is modified by the quantum coherence due to nonzero PDMs. Moreover, the reflected probe light field for both transitions experiences superluminal light propagation. Goos–Hanchen shifts of the reflected probe light beam are substantially modified by the refractive index of the interactive medium as well as incident angle. The effect of the coherent coupling field intensity on absorption, dispersion, group index, and the Goos–Hanchen shifts of the reflected probe beam is also discussed.

Natural Bond Order Analysis of HCN→HNC Isomerization Mechanism in the Interstellar Clouds Conditions: DFT Investigation

Natural Bond Order Analysis of HCN→HNC Isomerization Mechanism in the Interstellar Clouds Conditions: DFT Investigation

Associative detachment (AD) paths for H and CN- in the gas-phase: astrophysical implications.

The direct dynamical paths leading to Associative Detachment (AD) in the gas-phase, and specifically in the low-temperature regions of the Dark Molecular Clouds (DMC) in the ISM, or in cold trap laboratory experiments, are investigated with quantum chemical methods by using a high-level multi-reference Configuration Interaction (CI) approach that employs single and double excitations plus Davidson perturbative correction [MRSDCI(Q)] and the d-aug-cc-pV5Z basis set. The potential energy curves for H + CN- are constructed for different directions of the H partner approaching the CN- anion within the framework of the Born-Oppenheimer approximation. The present calculations found that the AD energetics at low temperature becomes favorable only along a selected range of approaching directions, thus showing that there is a preferred path of forming HCN at low temperatures, while that of forming its HNC isomer is found to be energetically forbidden. Given the existence in the ISM of different HCN/HNC ratios in different environments, we discuss the implications of our findings for selective formation of either isomer in the low-temperature conditions of the molecular cloud cores.

The Photodissociation of HCN and HNC: Effects on the HNC/HCN Abundance Ratio in the Interstellar Medium.

The impact of the photodissociation of HCN and HNC isomers is analyzed in different astrophysical environments. For this purpose, the individual photodissociation cross section of HCN and HNC isomers have been calculated in the 7-13.6 eV photon energy range for a temperature of 10 K. These calculations are based on the ab initio calculation of three-dimensional adiabatic potential energy surfaces of the 21 lower electronic states. The cross sections are then obtained using a quantum wave packet calculation of the rotational transitions needed to simulate a rotational temperature of 10 K. The cross section calculated for HCN shows significant differences with respect to the experimental one, and this is attributed to the need of considering non-adiabatic transitions. Ratios between the photodissociation rates of HCN and HNC under different ultraviolet radiation fields have been computed by renormalizing the rates to the experimental one. It is found that HNC is photodissociated faster than HCN by a factor of 2.2, for the local interstellar radiation field, and 9.2, for the solar radiation field at 1 au. We conclude that to properly describe the HNC/HCN abundance ratio in astronomical environments illuminated by an intense ultraviolet radiation field it is necessary to use different photodissociation rates for each of the two isomers, obtained by integrating the product of the photodissociation cross sections and ultraviolet radiation field over the relevant wavelength range.

Saddle point localization of molecular wavefunctions.

The quantum mechanical description of isomerization is based on bound eigenstates of the molecular potential energy surface. For the near-minimum regions there is a textbook-based relationship between the potential and eigenenergies. Here we show how the saddle point region that connects the two minima is encoded in the eigenstates of the model quartic potential and in the energy levels of the [H, C, N] potential energy surface. We model the spacing of the eigenenergies with the energy dependent classical oscillation frequency decreasing to zero at the saddle point. The eigenstates with the smallest spacing are localized at the saddle point. The analysis of the HCN ↔ HNC isomerization states shows that the eigenstates with small energy spacing relative to the effective (v1, v3, ℓ) bending potentials are highly localized in the bending coordinate at the transition state. These spectroscopically detectable states represent a chemical marker of the transition state in the eigenenergy spectrum. The method developed here provides a basis for modeling characteristic patterns in the eigenenergy spectrum of bound states.

Photodissociation of HCN and HNC isomers in the 7-10 eV energy range.

The ultraviolet photoabsorption spectra of the HCN and HNC isomers have been simulated in the 7-10 eV photon energy range. For this purpose, the three-dimensional adiabatic potential energy surfaces of the 7 lowest electronic states, and the corresponding transition dipole moments, have been calculated, at multireference configuration interaction level. The spectra are calculated with a quantum wave packet method on these adiabatic potential energy surfaces. The spectra for the 3 lower excited states, the dissociative electronic states, correspond essentially to predissociation peaks, most of them through tunneling on the same adiabatic state. The 3 higher electronic states are bound, hereafter electronic bound states, and their spectra consist of delta lines, in the adiabatic approximation. The radiative lifetime towards the ground electronic states of these bound states has been calculated, being longer than 10 ns in all cases, much longer that the characteristic predissociation lifetimes. The spectra of HCN is compared with the available experimental and previous theoretical simulations, while in the case of HNC there are no previous studies to our knowledge. The spectrum for HNC is considerably more intense than that of HCN in the 7-10 eV photon energy range, which points to a higher photodissociation rate for HNC, compared to HCN, in astrophysical environments illuminated by ultraviolet radiation.

Structure and properties of the radiation-induced intermediates produced from HCN in noble gas matrices

In this work we report the results of systematic studies on the radiation-induced transformations in HCN/Ng systems (Ng=Ne, Ar, Kr or Xe) at 7K using a combination of FTIR and EPR spectroscopy. It was shown that HCN underwent efficient decomposition producing H atoms, CN radicals and HNC isomer. The thermally induced reactions of H atoms in different matrices result in the formation of two isomeric radicals, H2CN and trans-HCNH, the former being predominated. The temperature dependent dynamics of CN and H2CN radicals in a krypton matrix was observed by EPR spectroscopy in solid krypton. The vibrational frequencies, IR intensities and magnetic resonance parameters of H2CN and trans-HCNH radicals calculated at the CCSD(T) level are in reasonable agreement with the experimental results. It was found that HCNH radical could be effectively bleached with visible light. The comparison of experimental and computational data made it possible to assign a new vibrational band at 918cm−1 in an Ar matrix (and the corresponding bands in Kr and Xe) to trans-HCNH radical. In addition, HKrCN was found in the case of krypton, whereas HXeCN and HXeNC were produced in solid xenon. The reaction mechanisms and contribution of different channels are discussed.

On the HCN - HNC Energy Difference.

The value for the HCN → HNC 0 K isomerization energy has been investigated by combining state-of-the-art electronic structure methods with the Active Thermochemical Tables (ATcT) approach. The directly computed energy difference between HCN and HNC at the HEAT-456QP level of theory is 5236 ± 50 cm(-1). This is substantially lower (by ∼470 cm(-1) or ∼1.3 kcal/mol) than the recently proposed high-level multireference configuration interaction value of 5705 ± 20 cm(-1) of Barber et al. ( Mon. Not. R. Astron. Soc. 2014, 437, 1828-1835 ). The discrepancy was analyzed by the ATcT approach, using several distinct steps, which (a) independently corroborated the current single-reference HEAT-456QP result, (b) independently found that the recent multireference-based value is highly unlikely to be correct within its originally stated uncertainty, and (c) produced a recommended value of 5212 ± 30 cm(-1) for the HCN → HNC isomerization energy at 0 K, based on all currently available knowledge. The ATcT standard enthalpies of formation at 0 and 298 K for HCN, HNC, and their cations and anions are also presented.

Selective Vibrational Population Transfer using Combined Stimulated Raman Adiabatic Passage and Counter-Diabatic Fields

We report studies of state-to-state vibrational energy transfer in an isolated polyatomic molecule driven by combined stimulated Raman adiabatic passage (STIRAP) and counter-diabatic fields (CDFs). The goal of these studies is development of a method for improving the efficiency of level-to-level population transfer under conditions that degrade the efficiency of conventional STIRAP-generated population transfer. The vehicles for our studies are selective population of one of a pair of near-degenerate states in nonrotating SCCl2 and enhancing the yield of nonrotating HNC in the HCN → HNC isomerization reaction. The efficiency of the population transfer within a subset of states embedded in a manifold of states is examined for the cases in which the transition dipole moments between the initial and target states are much smaller than and comparable to the transition dipole moments between background states and for the case in which the subset states have large transition dipole moments with the background ...

Highly excited rovibrational states of HNC

The [H, C, N] molecular system is a very important model system to many fields of chemical physics. The experimental characterization of highly excited vibrational states of this molecular system with a stable HCN and a metastable HNC isomer is of special interest. This work substantially extends the spectroscopic characterization of the rovibrational states of the [H, C, N] molecular system around the HNC minimum of the potential surface. The emission spectrum of the HNC isomer has been recorded at 1463 K using hot gas molecular emission (HOTGAME) spectroscopy from the far infrared up to the near infrared wavenumber region. The dense emission spectrum was analyzed using the spectrum analysis software SyMath™ implemented in the Mathematica™ computer algebra system. This work reports the analysis of highly excited rovibrational states. The rotational structure for 27 vibrational states ( 0 v 2 l 1 states from v 2 = 4 to v 2 = 7, the 0 v 2 l 2 from v 2 = 1 to v 2 = 4 and 2 v 2 l 0 states from v 2 = 3 to v 2 = 4) have been characterized for the first time and for five known vibrational levels it was possible to improve the existing spectroscopic constants substantially. Together with the author’s three recent papers accurate spectroscopic constants for 94 HNC rovibrational states have been determined. New expansion coefficients for the vibrational dependence of the spectroscopic constants are reported from the analysis of all these states.

Dipole-Resonance Assisted Isomerization in the Electronic Ground State Using Few-Cycle Infrared Pulses

A computational investigation of HCN → HNC isomerization in the electronic ground state by one- and few-cycle infrared pulses is presented. Starting from a vibrationally pre-excited reagent state, isomerization yields of more than 50% are obtained using single one- to five-cycle pulses. The principal mechanism includes two steps of population transfer by dipole-resonance (DR), and hence, the success of the method is closely linked to the polarity of the system and, in particular, the stepwise change of the dipole moment from reactant to transition state and on to products. The yield drops massively if the diagonal dipole matrix elements are artificially set to zero. In detail, the mechanism includes DR-induced preparation of a delocalized vibrational wavepacket, which traverses the barrier region and is finally trapped in the product well by DR-dominated de-excitation. The excitation and de-excitation steps are triggered by pulse lobes of opposite field direction. As the number of optical cycles is increased, the leading field lobes prepare a vibrational superposition state by off-resonant ladder climbing, which is then subjected to the three steps of the principal isomerization mechanism. DR excitation is more efficient from a preformed vibrational wavepacket than from a molecular eigenstate. The entire process can be loosely described as Tannor-Kosloff-Rice type transfer mechanism on a single potential surface effected by a single pulse, individual field lobes assuming the roles of pump- and dump-pulses. Pre-excitation to a transient wavepacket can be enhanced by applying a separate, comparatively weak few-cycle prepulse, in which the prepulse prepares a vibrational wavepacket. The two-pulse setup corresponds to a double Tannor-Kosloff-Rice control scheme on a single potential surface.

HNC/HCN Ratio in Acetonitrile, Formamide, and BrCN Discharge

Time-resolved Fourier transform (FT) spectrometry was used to study the dynamics of radical reactions forming the HCN and HNC isomers in pulsed glow discharges through vapors of BrCN, acetonitrile (CH(3)CN), and formamide (HCONH(2)). Stable gaseous products of discharge chemistry were analyzed by selected ion flow tube mass spectrometry (SIFT-MS). Ratios of concentrations of the HNC/HCN isomers obtained using known transition dipole moments of rovibrational cold bands v(1) were found to be in the range 2.2-3%. A kinetic model was used to assess the roles the radical chemistry and ion chemistry play in the formation of these two isomers. Exclusion of the radical reactions from the model resulted in a value of the HNC/HCN ratio 2 orders of magnitude lower than the experimental results, thus confirming their dominant role. The major process responsible for the formation of the HNC isomer is the reaction of the HCN isomer with the H atoms. The rate constant determined using the kinetic model from the present data for this reaction is 1.13 (±0.2) × 10(-13) cm(3) s(-1).

The rotational excitation of the interstellar HNC by para- and ortho-H2

Rotational excitation of the interstellar HNC due to collisions with H(2) is investigated. We present a new four dimensional (4D) potential energy surface for the HNC-H(2) collisional system. Both molecules were treated as rigid rotors. Interaction energy was obtained from the electronic structure calculations using a single and double-excitation coupled cluster method with perturbative contributions from connected triple excitations [CCSD(T)]. The five atoms were described using the aug-cc-pVTZ basis sets. Bond functions were placed at mid-distance between the HNC center of mass and the center of mass of H(2) for a better description of the van der Waals interaction. Close coupling calculations of the inelastic integral cross sections of HNC in collisions with para-H(2) and ortho-H(2) were calculated for kinetic energies up to 800 cm(-1). After Boltzmann thermal averaging, rate coefficients were obtained for temperatures ranging from 5 to 100 K. Significant differences exist between para- and ortho-H(2) results. The strongest collision-induced rotational HNC transitions are the transitions with Δj = 1 for collisions with para-H(2) and with ortho-H(2). The new rate coefficients should induce important consequences on the determination of HNC abundance in the interstellar medium. In particular, we expect that they will help to solve the interstellar problem of relative abundance of the HCN and HNC isomers.

Efficient geometry optimization by Hellmann–Feynman forces with the anti-Hermitian contracted Schrödinger equation

An efficient method for geometry optimization based on solving the anti-Hermitian contracted Schrödinger equation (ACSE) is presented. We formulate a reduced version of the Hellmann–Feynman theorem (HFT) in terms of the two-electron reduced Hamiltonian operator and the two-electron reduced density matrix (2-RDM). The HFT offers a considerable reduction in computational cost over methods which rely on numerical derivatives. While previous geometry optimizations with numerical gradients required 2M evaluations of the ACSE where M is the number of nuclear degrees of freedom, the HFT requires only a single ACSE calculation of the 2-RDM per gradient. Synthesizing geometry optimization techniques with recent extensions of the ACSE theory to arbitrary electronic and spin states provides an important suite of tools for accurately determining equilibrium and transition-state structures of ground- and excited-state molecules in closed- and open-shell configurations. The ability of the ACSE to balance single- and multi-reference correlation is particularly advantageous in the determination of excited-state geometries where the electronic configurations differ greatly from the ground-state reference. Applications are made to closed-shell molecules N2, CO, H2O, the open-shell molecules B2 and CH, and the excited state molecules N2, B2, and BH. We also study the HCN ↔ HNC isomerization and the geometry optimization of hydroxyurea, a molecule which has a significant role in the treatment of sickle-cell anaemia.

The [formula omitted] band system of HNC

The [H,C,N] molecular system is a very important model system to many fields of chemical physics. The experimental characterization of highly excited vibrational states of this molecular system with a stable HCN and a metastable HNC isomer is of special interest. The emission spectrum of the HNC isomer has been recorded at 1463 K using hot gas molecular emission (HOTGAME) spectroscopy in the wavenumber region of 3400 – 3800 cm - 1 with a resolution of 0.01 cm - 1 . The dense emission spectrum was analyzed with the spectrum analysis software SyMath™ implemented in the Mathematica™ computer algebra system. This work reports the analysis of the 1 v 2 l 0 → 0 v 2 l 0 band series up to v 2 = 7 and of the 1 v 2 l 1 → 0 v 2 l 1 band series up to v 2 = 4 . 15 rovibronic substates of HNC including all l = 1 , 3 , 5 , 7 sublevels of the v 1 v 2 l v 3 = 17 l 0 highly excited bending combination mode have been characterized for the first time and for the 14 known vibrational sublevels it was possible to improve the existing spectroscopic constants substantially. 23 vibrational sublevels are located for the first time relative to the 00 0 0 state. The exact position of these vibrational levels is an important contribution of this work to the spectroscopic characterization of the bending states of the [H,C,N] molecular system around the HNC minimum of the potential surface.

Molecular Geometries and Harmonic Frequencies from the Parametric Two-Electron Reduced Density Matrix Method with Application to the HCN ↔ HNC Isomerization

Energies, geometries, and harmonic frequencies of the chemical species in the HCN < = > HNC isomerization including the transition state are computed with the parametric variational two-electron reduced density matrix (2-RDM) method. The parametric 2-RDM method parametrizes the 2-RDM with single- and double-excitation coefficients to be both size-consistent and nearly N-representable [DePrince, A. E., III; Mazziotti, D. A. Phys. Rev. A 2007, 73, 042501.]. With the inclusion of the zero-point energies of both species, the energy of HNC relative to HCN in a polarized valence triple-zeta basis set is 14.2 kcal/mol, which agrees with the experimental value of 14.8 +/- 2 kcal/mol. The present calculations provide the first assessment of the method for computing harmonic frequencies from a molecular geometry optimization. Bond lengths, angles, and harmonic frequencies are also computed for HF, CO, and H2O.

Evolution of Chemical Bonding during HCN⇄HNC Isomerization as Revealed through Nuclear Quadrupole Hyperfine Structure

Hyperfine reporter: Millimeter-wave absorption measurements and ab initio calculations show that nuclear quadrupole hyperfine structure (hfs) is highly sensitive to the extent of bending excitation in HCN and HNC. These results indicate that hfs serves as a reporter of progress along the isomerization reaction coordinate and can reveal mechanistic information about this process.