On spectroscopic properties and isotope effects of vibrationally stabilized molecules

Results of quantum and semiclassical calculations obtained for two different potential-energy surfaces are used to discuss spectroscopic properties and isotope effects of the linear IHI and IDI molecules. The potentials are a purely repulsive LEPS surface and a DIM-3C potential with two van der Waals type minima for equivalent IH ··· I and I ··· HI configurations. Both systems are dominated by the effect of vibrational bonding giving rise to some very unusual spectroscopic phenomena, which are discussed in detail. The different vibrational frequencies and rotational constants are roughly estimated as ν₁ = 120 (100) cm^?1, ν₂ = 280 (210) cm^?1, ν₃ = 360 (160) cm^?1 and B = 0.0194 (0.0196) cm^?1 for IHI (IDI). A detailed discussion of the dependence of ν₁, ν₂ and B on ν₃, their sensitivity to variations of the potential-energy surface, and a comparison with the vibrational frequencies of I₂ and HI (ID) is given. It is predicted that there exists only one excited level of the antisymmetric stretching mode. The numbers of symmetrical stretching and bending levels are fairly constant or may even decrease upon deuteration. Simultaneously deuteration destabilizes the molecule. These unusual phenomena are rationalized by our calculations. A set of criteria for observing infrared and Raman bound-to-bound and bound-to-resonance state transitions are presented for the IHI and IDI molecule.     

C30卡宾三叶结分子结构与稳定性的理论研究

三叶结分子是最简单的非平凡纽结分子, C₃₀卡宾三叶结分子是由一条闭合的(C≡C—)₁₅ sp杂化碳链组成的, 是具有D₃对称性的拓扑手性分子. 本文用密度泛函方法[DFT/RB3LYP/6-31G(D)]对分子结构和光谱性质进行了研究, 在优化构型的基础上通过自然键轨道(NBO)方法和轨道能级研究了它的共轭性、成键情况和稳定性, 并与平面型C₃₀卡宾环分子进行了比较. 计算结果表明三叶结分子的三叶弧上形成了非平面的C≡C共轭和扭曲的内螺旋结构, 交叉处具有弱成键作用, 且分子轨道也发生了扭曲; 三叶结分子比卡宾环的共轭性和赝Jahn-Teller效应都明显小, 而总能量高. 因此, 分子打结是一个能量升高的过程.     

Synthesis,mass spectra,raman and infrared spectra of the compounds M(PF3)6 (M = Cr,Mo, W)

An improved method of synthesis of hexakis(trifluorophosphine)chromium(0), -molybdenum(0)and -tungsten(0) is described, involving photochemically induced substituion of carbonyl groups from the (trifluorophosphine)tricarbonylmetallates by PF₃ under moderate pressure. A full analysis of the fragmentation patterns in a mass spectrometer is given, and both Raman and infrared data obtained throughoutthe range 4000–50 cm^?1 are discussed. Complete vibrational mode assignments are made, and a normal coordinate analysis leading to the determination of force constants and potential energy distributions is presented. The normal mode in which M-P bond stretching is predominant in all cases involves a significant contribution from PF bond stretching in these molecules.     

Imines in the Titanium Coordination Sphere – Transformation of Imidoyl Chlorides to Nitrilium Ions

In the reaction of TiCl₄ in benzene as solvent with the imidoyl chloride p‐Tolyl(Cl)C=NPh ( 1 ) the abstraction of the chloride substituent is observed, leading to the nitrilium salt [p‐Tolyl–C≡N–Ph]⁺[Ti₂Cl₉]^– ( 2 ) in quantitative yield. The highly electrophilic salt 2 is characterized by IR‐ and NMR spectroscopy. The observed band for the C≡N stretching mode of 2 clearly indicates the formation of a nitrilium ion. Especially a characteristic line broadening of the ¹³C{¹H}‐NMR signals related to carbon atoms next to the nitrogen is observed. By ¹⁵N,¹H‐HMBC NMR experiments it is shown that the nitrogen signal of 2 is significantly shifted to high‐field in relation to nitriles and imines. The molecular structure of 2 was confirmed by single‐crystal X‐ray diffraction. The C≡N bond length and the linearity of the C–C≡N–C unit in 2 confirm the triple bond character of this bond.     

A quasiclassical trajectory study of vibrational energy transfer in collisions involving intermolecular attraction of moderate strength

Monte Carlo selected, quasiclassical trajectories have been computed on six potential energy hypersurfaces possessing potential minima or “wells” up to 50 kJ mol^?1 deep. The aim of the investigation has been to examine how vibrational energy transfer in A + BC(υ = 1) collisions is promoted by intermolecular attraction of moderate strength. Here results are reported for the mass combination m_A = 20 u, m_B = 1 u, m_C = u. The results show that even quite slight intermolecular attraction can enhance energy transfer, as long as the attraction does not just depend on the separation of A from the center-of-mass of BC. The mean loss of vibrational energy does not depend only the well depth but also on its “location” (in particular, the difference in r_BC at the minimum and in isolated BC) and on the angular anisotropy of the potential. Large transfers of energy do not occur only in complex-forming collisions; indeed, a high fraction of trajectories on all surfaces are direct but show similar transfer of energy as in the more complex trajectories on the same surface. The results of the calculations are discussed in relation to the mechanisms and rates of vibrational relaxation in collisions between radicals and between species. such as HF + HF, capable of forming hydrogen bonds.     

Shedding Light on the Vibrational Signatures in Halogen-Bonded Graphitic Carbon Nitride Building Blocks

The relative contributions of halogen and hydrogen bonding to the interaction between graphitic carbon nitride monomers and halogen bond (XB) donors containing C−X and C≡C bonds were evaluated using computational vibrational spectroscopy. Conventional probes into select vibrational stretching frequencies can often lead to disconnected results. To elucidate this behavior, local mode analyses were performed on the XB donors and complexes identified previously at the M06-2X/aVDZ-PP level of theory. Due to coupling between low and high energy C−X vibrations, the C≡C stretch is deemed a better candidate when analyzing XB complex properties or detecting XB formation. The local force constants support this conclusion, as the C≡C values correlate much better with the σ-hole magnitude than their C−X counterparts. The intermolecular local stretching force constants were also assessed, and it was found that attractive forces other than halogen bonding play a supporting role in complex formation.     

Density functional studies of AnF6 (An=U,Np, and Pu) and UF6−nCln (n=1–6) using hybrid functionals: geometries and vibrational frequencies

We have compared the performance of widely used hybrid functionals for calculating the bond lengths and harmonic vibrational frequencies of AnF₆ (An=U, Np, and Pu) and UF_6?nCl_n (n=1–6) molecules using “small‐core” relativistic effective core potentials and extended basis sets. The calculated spectroscopic constants compare favorably with experimental data for the bond lengths (average error ≤ 0.01 Å) and vibrational frequencies (average error ≤ 7 cm^?1) of the AnF₆ molecules. The experimental vibrational frequencies of the stretching modes were available for most of the UF_6?nCl_n (n=1–6) molecules. The calculated vibrational frequencies are in good agreement with the experimental data to within 4.6 cm^?1 and 7.6 cm^?1 for selected Becke1 and Lee, Yang, Parr (B1LYP), and Becke3 and Perdew, Wang (B3PW91) functionals, respectively. We conclude that one can predict reliable geometries and vibrational frequencies for the unknown related systems using hybrid density functional calculations with the RECPs. The geometries and vibrational frequencies of the UF_6?nCl_n (n=1–6) molecules that have not been determined experimentally are also presented and discussed. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 2010–2017, 2001     

Ab initio molecular dynamics study for C–Br dissociation within BrH<Subscript>2</Subscript>C–C≡C<Subscript>(ads)</Subscript> adsorbed on an Ag(111) surface: a short-time Fourier transform approach

The reaction dynamics for C–Br dissociation within BrH₂C–C≡CH_(ads) adsorbed on an Ag(111) surface has been investigated by combining density functional theory-based molecular dynamics simulations with short-time Fourier transform (STFT) analysis of the dipole moment autocorrelation function. Two possible reaction pathways for C–Br scission within BrH₂C–C≡CH_(ads) have been proposed on the basis of different initial structural models. Firstly, the initial perpendicular orientation of adsorbed BrH₂C–C≡CH_(ads) with a stronger C–Br bond will undergo dynamic rotation leading to the final parallel orientation of BrH₂C–C≡CH_(ads) to cause the C–Br scission, namely, an indirect dissociation pathway. Secondly, the initial parallel orientation of adsorbed BrH₂C–C≡C_(ads) with a weaker C–Br bond will directly cause the C–Br scission within BrH₂C–C≡CH_(ads), namely, a direct dissociation pathway. To further investigate the evolution of different vibrational modes of BrH₂C–C≡CH_(ads) along these two reaction pathways, the STFT analysis is performed to illustrate that the infrared (IR) active peaks of BrH₂C–C≡CH_(ads) such as vCH₂ [2956 cm^?1(s) and 3020 cm^?1(as)], v≡CH (3320 cm^?1) and vC≡C (2150 cm^?1) gradually vanish as the rupture of C–Br bond occurs and then the resulting IR active peaks such as C=C=C (1812 cm^?1), ω-CH₂ (780 cm^?1) and δ-CH (894 cm^?1) appear due to the formation of H₂C=C=CH_(ads) which are in a good agreement with experimental reflection adsorption infrared spectrum (RAIRS) at temperatures of 110 and 200 K, respectively. Finally, the total energy profiles indicate that the reaction barriers for the scission of C–Br within BrH₂C–C≡CH_(ads) along both direct and indirect dissociation pathways are very close due to a similar rupture of C–Br bond leading to a similar transition state.     

Structures and stability of SCBO+/− and SBCO+/−: prediction of very short yet classical triple bonding of sulfur

Compounds containing the sulfur triple bonding have continued to attract chemists’ attention. In this article, with an attempt to predict intrinsically stable species with sulfur-related triple bonding, we report a thorough computational study of two charged systems [B,C,O,S]⁺ and [B,C,O,S]^? at the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311+G(3df,2p)+ZPVE level for singlet and triplet potential energy surfaces, aug-cc-pVTZ-B3LYP, M06-2X, and CCSD(T) levels for critical structures, as well as the CCSD(T)/aug-cc-pVQZ and G4 levels for adiabatic bond dissociation energy (ABDE). A total of 26 isomers and 25 transition states were located. The cationic and anionic [B,C,O,S] have the singlet and triplet ground states, respectively. For both systems, the former low-lying isomers are the linear SCBO^+/? 01 and SBCO^+/? 02, both of which contain the S≡X (X = C, B) bonding and are kinetically very stable against interconversion and fragmentation. With the increased valence electron number in the order of SCBO/SBCO⁺ < SCBO/SBCO < SCBO/SBCO^?, the SX bond distance elongates as 1.5653 < 1.6126 < 1.6924 Å for X = B and 1.4715 < 1.5319 < 1.6100 Å for X = C at the M06-2X/aug-cc-pVTZ level. Notably, SBCO⁺ bears the shortest S≡B bond known to date, while SCBO⁺ bears the shortest S≡C bond among the known classical and non-protonated compounds (the well-known F₃SCCF₃ and F₃SCSF₅ have been termed as “nonclassical” because of their unusually low ABDE of SC bond). Future mass spectroscopic studies are greatly appealed for the characterization of the cationic and anionic SCBO^+/? 01 as well as SBCO^+/? 02.     

Bond types of molecular orbitals and the photoelectron spectrum

The vibrational structures of the photoelectron spectra for diatomic molecules can be accounted for in terms of the slope of the orbital energy curve in the conventional correlation diagram with respect to internuclear distance. The vibrational structures of the photoelectron spectra for simple polyatomic molecules HCN, C₂H₂, and AH₂ type of hydrides can also be accounted for in terms of the slopes of the orbital energy curves in the correlation diagrams with respect to angles, as well as distances. Among all correlation diagrams, the slopes in the distance correlation diagram are related to the criterion for bond type—the positive for “bonding,” the negative for “antibonding,” while slopes with small magnitudes for “nonbonding.” The Fock matrix elements within the bond orbital basis provide heuristic and systematic rationalization of the slopes for the orbital energy curves. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 81: 53–65, 2001     

Correlation between intramolecular bond distances and stretching vibrations for polar molecules: An ab initio study

The correlation between intramolecular bond length and vibrational frequency shifts was calculated at the MP4(aug-cc-PVTZ) ab initio level for a number of molecules (LiH, BH, HF, OH⁻, HDO, BF, CN⁻, and HCI) exposed to uniform electric fields in the range from −0.10 to +0.10 au. The “ω vs. r_e” correlation curves always consist of two branches, each approximately linear. The slopes for the molecules investigated here vary between −2500 and −16600 cm⁻¹/Å. The slopes are well described by an expression containing only the free-molecule second- and third-order force constants and the reduced mass for the stretching mode. Experimental data for polar molecules can be expected to show deviations from a linear “ω vs. r_e” correlation (i) for molecules where the maximum of the frequency vs. field curve occurs at a positive field and (ii) for molecules where the maximum of the frequency vs. field curve falls on the negative-field side but very close to the zero-field case, and (iii) in bonding situations when there is much electron overlap. As opposed to uniform-field situations, anharmonicity and electronic overlap have a substantial influence on the “frequency vs. r_e” slopes in molecular environments. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 537–546, 1997     

Spectra and structure of phosphorus-boron compounds: VII. Vibrational analysis of solid phosphine-borane

The infrared (70–2700 cm^?) and Raman (25–2500 cm^?1) spectra of H₃PBH₃, H₃PBD₃, D₃PBH₃ and D₃PBD₃ in the solid state at ?196 °C have been recorded. The shift associated with the boron-10 and boron-11 isotopes was observed for the P-B stretching motion. A complete vibrational assignment is proposed and a normal coordinate calculation utilizing a valence force field model has been carried out. The force constant of 1.97 mdyn Å^?1 for the phosphorus-boron stretching mode is consistent with the relatively long phosphorus-boron bond; this constant is compared to the similar quantity for several other phosphorus-boron compounds. None of the E modes for the “free” molecule were found to be split. The number of observed lattice modes is not consistent with the crystal structure previously reported for this molecule. A possible explanation is discussed.     

A Spectroscopic Study on Nitrogen Electrooxidation to Nitrate

As a potential substitute technique for conventional nitrate production, electrocatalytic nitrogen oxidation reaction (NOR) is gaining more and more attention. But, the pathway of this reaction is still unknown owing to the lack of understanding on key reaction intermediates. Herein, electrochemical in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and isotope-labeled online differential electrochemical mass spectrometry (DEMS) are employed to study the NOR mechanism over a Rh catalyst. Based on the detected asymmetric NO₂⁻ bending, NO₃⁻ vibration, N=O stretching, and N−N stretching as well as isotope-labeled mass signals of N₂O and NO, it can be deduced that the NOR undergoes an associative mechanism (distal approach) and the strong N≡N bond in N₂ prefers to break concurrently with the hydroxyl addition in distal N.     

Osmium(II)-Phthalocyanine: Darstellung und Eigenschaften von Di(acido)phthalocyaninatoosmaten(II)

Osmium(II) Phthalocyanines: Preparation and Properties of Di(acido)phthalocyaninatoosmates(II) “H[Os(X)₂Pc^2?]” (X = Br, Cl) reacts in basic medium or in the melt with (ⁿBu₄N)X forming less stable, diamagnetic, darkgreen (ⁿBu₄N)₂[Os(X)₂Pc^2?]. Similar dicyano and diimidazolido(Im) complexes are formed by the reaction of “H[Os(Cl)₂Pc^2?]” with excess ligand in the presence of [BH₄]^?. The cyclic voltammograms show up to three quasireversible redoxprocesses: E_1/2(I) = 0.13 V (X = CN), ?0.03 V (Im), ?0.13 V (Br) resp. ?0.18 V (Cl) is metal directed (Os^II/III), E_1/2(II) = 0.69 V (Cl), 0.71 V (Br), 0.83 V (CN), 1.02 V (Im) is ligand directed (Pc^2?/?) and E_1/2(III) = 1.17 V (Cl) resp. 1.23 V (Br) is again metal directed (Os^III/IV). Between the typical “B” (～16.2 kK) and “Q” (～29.4 kK), “N regions” (～34.1 kK) up to seven strong “extra bands” of the phthalocyanine dianion (Pc^2?) are observed in the uv-vis spectrum. Within the row CN > Im > Br > Cl, most of the bands are shifted slightly, the “extra bands” considerably more to lower energy in correlation with E_1/2(I). The vibrational spectra are typical for the Pc^2? ligand with D_4h symmetry. M.i.r. bands at 514, 909, 1 173 and 1 331 cm^?1 are specific for hexa-coordinated low spin Os^II phthalocyanines. In the resonance Raman (r.r.) spectra polarized, depolarized or anomalously polarized deformation and stretching vibrations of the Pc^2? ligand will be selectively enhanced, if the excitation frequency coincides with “extra bands”. With excitation at ～19.5 kK the intensity of the symmetrical Os? X stretching vibration at 295 cm^?1 (X = Cl), 252 cm^?1 (X = Im) and 181 cm^?1 (X = Br) is r.r. enhanced, too. The asymmetrical Os? X stretching vibration is observed in the f.i.r. spectrum at 345 cm^?1 (X = CN), 274 cm^?1 (X = Cl), 261 cm^?1 (X = Im) and 200 cm^?1 (X = Br).     

On a possible mechanism for Ar 4 + fragmentation

The full potential energy surface (PES) for the collinear Ar ₄ ⁺ cluster as a function of the three internuclear distances is computed at the post-Hartree-Fock level using Density Functional Theory (DFT) methods to treat dynamic correlation effects. The behaviour of the overall configuration energy minima as the central Ar ₂ ⁺ bond stretches is analysed as a function of the fragmentation coordinates of the wing atoms. The coupling between the stretching coordinate and the fragmentation coordinates is also analysed over the whole PES. The calculations suggest that large vibrational energy content in the core dimer ion causes localization of the coupling with either wing atoms which could in turn favour energetically the sequential fragmentation, while Ar ₄ ⁺ with a vibrationally cold core markedly lowers any energy barrier to fragment in a concerted fashion. Such suggestions provide further useful information for what has been found in some of the experimental studies on this ionic system (and on larger ionized argon clusters) and underline the possible role which the internal vibrational energy content of the ionic cluster can play in the fragmentation.     

Gas‐Phase Infrared Spectroscopy of Substituted Cyanobutadiynes: Roles of the Bromine Atom and Methyl Group as Substituents

The IR spectra of 5‐bromo‐2,4‐pentadiynenitrile (Br?C≡C?C≡C?CN) and 2,4‐hexadiynenitrile (CH₃?C≡C?C≡C?CN), a compound of interstellar interest, have been recorded within the 4000–500 cm^?1 spectral region and calculated by means of high‐level ab initio and density functional calculations. Although the calculated structures of both compounds are rather similar, there are very subtle differences, mainly in the strength of the C≡C bond not directly bound to the substituent. These subtle bonding differences are reflected in small, but not negligible, differences in the electron density at the corresponding bond critical points, and, more importantly, are reflected in the IR spectra. Indeed, the IR spectrum for the bromine derivative presents two well‐differentiated strong bands around 2250 cm^?1, whereas for the methyl derivative both absorptions coalesce in a single band. These bands correspond in both cases to the coupling between C≡C and C≡N stretching displacements. A third, very weak, band also associated with C≡C and C≡N coupled stretches is observed for the bromine derivative, but not for the methyl one, owing to its extremely low intensity.     

Vibrational spectra and reinvestigation of the crystal structure of a polymeric copper(II)-orotate complex, [Cu(μ-HOr)(H2O)2]n: The performance of new DFT methods, M06 and M05-2X, in theoretical studies

The crystal and molecular structure of a polymeric Cu(II)-orotate complex, [Cu(μ-HOr)(H₂O)₂]_n, has been reinvestigated by single crystal X-ray diffraction. It is shown that several synergistic interactions: two axial Cu-O interactions; intramolecular and intermolecular hydrogen bonds; and π-π stacking between the uracil rings contribute to the stability of the crystal structure. The Raman and FT-IR spectra of the title complex are reported for the first time. Comprehensive theoretical studies have been performed by using three unrestricted DFT methods: B3LYP; and the recently developed M06, and M05-2X density functionals. Clear-cut assignments of all the bands in the vibrational spectra have been made on the basis of the calculated potential energy distribution, PED. The very strong Raman band at 1219 cm⁻¹ is diagnostic for the N1-deprotonation of the uracil ring and formation of the copper-nitrogen bond, in this complex. The Cu-O (carboxylate) stretching vibration is observed at 287 cm⁻¹ in the IR spectrum, while the Cu-N (U ring) stretching vibration is assigned to the strong Raman band at 263 cm⁻¹. The molecular structure and vibrational spectra (frequencies and intensities) calculated by the M06 functional method are very similar to the results obtained by the B3LYP method, but M06 performs better than B3LYP in calculations of the geometrical parameters and vibrational frequencies of the interligand O-H?O hydrogen bonding. Unfortunately, the M05-2X method seriously overestimates the strength of interligand hydrogen bond.     

Vibrational relaxation of compressed nD2 fluid

Experimental observations of the vibrational population relaxation time of ⁿD₂ fluid under pressures of up to 500 atm in the 25–85 K range are presented and described in terms of a semi-classical model for energy transfer in liquids. For comparison with the parameters of this model, a classical equivalent potential for quantum systems is derived from the “real” intermolecular potential.     

Vibrational spectrum of Li3 first‐excited electronic doublet state: Geometric‐phase effects and statistical analysis

We present J=0 calculations of all bound and pseudobound vibrational states of Li₃ in its first‐excited electronic doublet state by using a realistic double many‐body expansion potential‐energy surface and a minimum‐residual filter diagonalization technique. The action of the system Hamiltonian on the wave function was evaluated by the spectral transform method in hyperspherical coordinates. Calculations of the vibrational spectra were carried out both without consideration and with consideration of geometric‐phase effects. Dynamic Jahn–Teller and geometric‐phase effects are found to play a significant role, while the calculated fundamental symmetric stretching frequency is larger by 8.3% than its reported experimental value of 326 cm⁻¹. From the neighbor‐spacing distributions of the levels, it is observed that the title vibrational spectrum is quasiregular in the short range and quasi‐irregular in the long range. By the Δ₂ standard defined in this article, it is found that the spectra are more nonuniform than those of the “trough” states for the ground electronic state. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 75: 89–109, 1999