Temperature-dependent, effective structures of the 14NH3 and 14ND3 molecules

Measurements result in effective, usually temperature-dependent structural parameters of molecules, and never directly in equilibrium structures, which are theoretical constructs. A recent high-accuracy semiglobal potential energy surface of the electronic ground state of the ammonia molecule, called NH3-Y2010 (J. Mol. Spectrosc. 2011, 268, 123), which exhibits mass-independent equilibrium NH bond length and a HNH bond angle of 1.0109 ? and 106.75°, respectively, is employed together with the variational nuclear motion code GENIUSH (J. Chem. Phys. 2009, 130, 134112; 2011, 134, 074105) to determine directly measurable, effective structural parameters of the (14)NH(3) and (14)ND(3) molecules. The effective r(g)- and r(a)-type NH(ND) distances determined at 300 K are 1.0307(1.0254) and 1.0256(1.0217) ?, respectively, with an estimated accuracy of 2 × 10(-4) ?. The effective θ(g) HNH and DND bond angles at 300 K are 106.91° and 106.85°, respectively. The root-mean-square amplitudes of vibration, l(g), for the NH(ND) distances at 300 K are 0.073(0.062) ?. These structural parameters confirm the less accurate results of a room-temperature gas-electron-diffraction study (J. Chem. Phys. 1968, 49, 2488, all data in ?): r(g)(NH) = 1.030(2), l(g)(NH) = 0.073(2), r(g)(ND) = 1.027(3), and l(g)(ND) = 0.061(2). The computed difference in the r(g,T)(NH) bond lengths of the two spin isomers (ortho and para forms) of (14)NH(3) is 3 × 10(-5) ? at 0 K, the difference diminishes at temperatures of about 30-50 K.     

Isotopic exchange of gaseous ammonia 14NH3 and 15NH3 in sulfonated macroreticular ion exchangers

Adsorption and ¹⁵NH₃ isotopic exchange was performed on dry macroreticular polystyrene ion exchanges crosslinked with varying amounts of divinylbenzene and partially neutralized by ¹⁴NH₃. Data on pressure changes and mass spectrometric analyses of isotopic composition of the gaseous phase were used to calculate equilibrium distribution of ¹⁴NH₃ and ¹⁵NH₃ under various dislocation conditions. It was established that along with the exchange of ¹⁴NH₃ to a gaseous phase, ¹⁵NH₃ penetrates to the mass of ion exchanger. This is evidently due to the migration of ammonia among functional groups. It was found that by thermal desorption under reduced pressure ammonia is released only from functional groups located on the surface of ion exchanger.     

Vibrational spectra,assignments and normal coordinate analysis of 3-aminobenzyl alcohol

The laser Raman and FTIR spectra of 3-aminobenzyl alcohol have been recorded. The observed frequencies were assigned to various modes of vibrations on the basis of normal coordinate analysis, assuming C(s) point group symmetry. The potential energy distribution associated with normal modes is also reported here. The assignment of fundamental vibrations agrees well with the calculated frequencies.     

Vibrational spectra,assignments and normal coordinate analysis for difluorophosphine

Infrared spectra of the gas and Raman spectra of the liquid and solid forms of PF₂H and PF₂D are reported. A normal coordinate analysis based on compliance constants has been carried out on the assigned fundamentals and also on harmonic frequencies estimated by Dennison's method.     

The ESR observation of 14NH+3 and 15NH+3 trapped in an argon matrix at 14 K

The molecular ion NH⁺₃ has been trapped in an argon matrix at 14 K by photoionisation of an NH₃/Ar mixture during deposition. The NH⁺₃ cation has been found to be rigidly trapped in the argon matrix and exhibits a powder ESR spectrum. The derived magnetic parameters agree well with those obtained by other workers in different matrices. A strong reversible temperature dependence of the linewidth of the NH⁺₃ spectrum has been observed.     

Vibrational spectra and dynamics of conformation and hydrogen bonding of N-methylacetamide. II. Dynamics of the NH...O Hydrogen bond and NH (ND) stretching band structure

The infrared and Raman spectra of crystalline CH₃CONHCH₃, CD₃CONHCH₃, CH₃CONHCD₃, CD₃CONHCD₃ and their N deuterated derivatives have been investigated at various temperatures in the 3400-2000 cm^1̄ range. A new interpretation of the NH (ND) stretching subbands is given and is based on the assumption of a strong coupling between NH stretching and deformation modes (γ NH and τ CN). Suitable potential functions are derived from experimental data and they allow to calculate the infrared and Raman spectra in the stretching region. The calculated spectra as well as the calculated isotopic and temperature shifts are in good agreement with the observed ones. The potential surface of the bending modes in the excited state has only one rather flat central minimum corresponding to a planar geometry of the amide group which can be contrasted to the four-minimum potential surface of the fundamental state.     

Theoretical study of the F + NH3 and F + ND3 reactions: mechanism and comparison with experiment

We study the barrierless and highly exothermic F + NH(3) and F + ND(3) abstraction reactions using quasiclassical trajectory calculations based on an analytical potential energy surface developed in our research group. The calculations correctly reproduce the experimental evidence that the vibrational fraction deposited into the DF product for the F + ND(3) reaction is greater than into HF for the F + NH(3) reaction and that the vibrational distribution is inverted in the HF(v') and the DF(v') products. Of special interest is that recent crossed-beam experiments reported by Yang and co-workers at 4.5 kcal mol(-1) are reproduced for both reactions, with a mainly forward symmetry associated with direct trajectories, and a small sideways-backward symmetry contribution associated with "nearly trapped" trajectories due to a "yo-yo" mechanism, different from the previously suggested mechanism of a long-lived complex.     

Kinetic and spectroscopic analysis of NH3 decomposition under R.F. Plasma at moderate pressures

The plasma decomposition of NH₃ has been studied as a function of the residence time, power input, and pressure. The process follows apparently zero-order kinetics, which can be interpreted on the basis of a kinetic mechanism involving as initial step the rupture of an N-H bond from vibro-rotationally excited modecules. Simultaneous spectroscopic observations of the emission light due to electronically excited NH₂, NH, H, and N₂ have been used to confirm the suggested mechanism and to show that NH₂ and NH are successive intermediate species and that the final step of the decomposition process is the bimolecular recombination NH+NHN₂+H₂.     

The generalized separated electron pair model. II. An application to NH,NH3, NH,NH2− and N3−

The Separated Electron Pair (SEP) model (Strongly Orthogonal Geminals) and methods for its systematic extension has been applied to the series: NH, NH₃, NH, NH^2? and N^3?. On going from NH to N^3?, the SEP model, in its most general form, recovers 85–75% of the intrapair correlation energy and 60–55% of the interpair correlation energy obtainable with the given basis set. The best wave functions for each species recovered about 45–50% of the total empirical correlation energy which is expected to be very close to the basis set limit. It was apparent that the SEP model yields a set of one-electron functions that are very useful for further improvement of the wave function. This fact apparently remains true even when the SEP model itself gives very poor energies.     

Synthesis, DTA, IR and Raman spectra of penthylenediammonium hexachlorostannate NH3(CH2)5NH3SnCl6.

The Raman and IR spectra of NH3(CH2)5NH3SnCl6 have been measured at ambient temperature. It is shown that the cations in the compound assume a symmetry lower than C2v. Combination bands observed in the 2100-1800 cm(-1) region in the IR spectrum of NH3(CH2)5NH3SnCl6 indicate that the compound contains the C-NH3 grouping, the bands are discussed and their assignment are suggested. No evidence of existence of hydrogen bonding is found from the infrared spectrum in the region of 2800-3200 cm(-1); anions and cations are found not connected by hydrogen bonding and are therfore isolated. The Raman spectrum of anions can be interpreted in terms of disordered groups, not clearly showing the predicted splitting of bands.     

Photoabsorption cross sections of NH3, NH2D, NHD2, and ND3 in the spectral range 110-144 nm

Cross sections for photoabsorption of NH3, NH2D, NHD2, and ND3 near 298 K were measured in the spectral range of 110-144 nm using radiation from a synchrotron. Absorption cross sections and oscillator strengths of NH3 agree satisfactorily with previous reports; those of ND3 are improved over those in a previous report, whereas those of NH2D and NHD2 are new. The oscillator strengths of transitions to D, D', D", F, and G states are nearly the same among all four isotopic variants, but those to D' and E states vary substantially. Observed absorption bands are arranged into vibrational progressions in accord with known Rydberg transitions. All progressions show a common trend of vibrational intervals increasing with vibrational quantum numbers. The Rydberg orbitals for states D(3de"), D'(4sa1'), D"(3da1'), D'(4pe'), and E(4de") are readily assigned with quantum defects determined in these experiments, but assignments for F(5de") and G(6de") are uncertain. Absorption cross sections of dissociative continua underneath discrete structures are larger for NH2D and NHD2 than for NH3 and ND3, indicating that the rate of dissociation of ammonia might increase when its symmetry is broken.     

Vibrational spectra,assignments and normal coordinate analysis of 2-amino-5-bromopyridine

The FTIR and laser Raman spectra of 2-amino-5-bromopyridine have been recorded. The observed frequencies were assigned to various modes of vibrations on the basis of normal coordinate calculations, assuming C(s) point group symmetry. The potential energy distribution associated with normal modes is also reported here. The assignment of fundamental vibrations agrees well with the calculated frequencies.     

FTIR and FT-Raman spectra, assignments, ab initio HF and DFT analysis of xanthine

The molecular vibrations of xanthine were investigated in polycrystalline sample, at room temperature by Fourier transform infrared (FTIR) and FT-Raman spectroscopies. The spectra of the molecule have been recorded in the regions 4000-50 cm(-1) and 3500-100 cm(-1), respectively. Theoretical information on the optimized geometry, harmonic vibrational frequencies, infrared and Raman intensities were obtained by means of ab initio Hartree-Fock (HF) and density functional theory (DFT) gradient calculations with complete relaxation in the potential energy surface using 6-311++G(d,p) basis set. The vibrational frequencies which were determined experimentally from the spectral data are compared with those obtained theoretically from ab initio and DFT calculations. A close agreement was achieved between the observed and calculated frequencies by refinement of the scale factors. The infrared and Raman spectra were also predicted from the calculated intensities. Thermodynamic properties like entropy, heat capacity, zero point energy have been calculated for the molecule. Unambiguous vibrational assignment of all the fundamentals was made using the potential energy distribution (PED).     

Raman spectra of (NH4)3ZnCl4NO3 and (ND4)3ZnCl4NO3 between 295 and 60 K

Raman spectra from polycrystalline samples of (NH4)3ZnCl4NO3 and (ND4)3ZnCl4NO3 have been studied in the temperature range 60-295 K. Internal modes of both nitrate and tetrachlorozincate ions show expected band narrowing and intensification at lower temperature but no significant changes in frequency. Two bands in the lattice region of both compounds, assigned to nitrate ion libration and rocking, show linear increases in frequency with lowering temperature. The intensity of the libration mode shows a linear decrease with lowering temperature, but the intensity of the rocking mode is relatively insensitive to temperature change. Ammonium ion bands show greater structure at low temperature, suggesting differentiation between the two crystallographically distinct types of cation. The observed spectral changes are interpreted on the basis of increasing ordering and effectiveness of hydrogen bonds between ammonium ions and nitrate ions at low temperatures. The Raman spectra give no evidence of discontinuous changes in frequency or intensity, which would signal temperature-dependent transitions of the crystal structure. Unlike the related single-anion compounds NH4NO3 and (NH4)2ZnCl4, the room-temperature structure of (NH4)3ZnCl4NO3 and (ND4)3ZnCl4NO3 appears to persist at least to 60 K, being stabilized by increasingly ordered hydrogen bonding.     

Study of vibrational spectra and their assignments for phenylphosphonic and phenylthiophosphonic acid and comparison to experiments

The structures and conformational stabilities of phenylphosphonic and phenylthiophosphonic acids are investigated using calculations mostly at the DFT/6-311G** level and ab initio ones at the MP2/6-311G** level (no frequency calculations in the latter case), because we know from our previous results that the addition of diffuse functions to a valence triple zeta basis with polarization functions might lead to an unbalanced basis set. Further, the experience tells that for large energy differences between conformers, DFT works very well. From the calculations the molecules are predicted to exist in a conformational equilibrium consisting of two non (near)-planar conformers that are identical by symmetry. Interestingly, in the internal rotation potential functions the planar conformer appears to be a stable minimum (also optimization converges to planar), however the vibrational frequencies were computed and the planar conformer exhibited an imaginary one, indicating that it is a maximum with respect to one of the internal coordinates. Only optimization without any restrictions and starting from a non (near)-planar structure converged to a real minimum with a non (near)-planar geometry. In the minimum structure, vibrational infrared and Raman spectra are calculated and those for phenylphosphonic acid are compared to experimental ones, showing satisfactory agreement. The rather low intensity of the OH bands in the experimental infrared spectrum (as compared to normal organic acids) indicates rather weak hydrogen bonding with at most dimers present. Normal coordinate calculations are carried out and potential energy distributions are calculated for the molecules in the non (near)-planar conformations providing a complete assignment of the vibrational modes to atomic motions in the molecules. From the rather low rotational barriers we conclude, in agreement with the results from the literature (for other P=O compounds) based on localized orbitals, that conjugation effects are absent — or at least negligible — as compared to electrostatic and steric ones.     

The vibrational spectra, assignments and ab initio/DFT analysis for 3-chloro, 4-chloro and 5-chloro-2-methylphenyl isocyanates

The Raman (3500-50 cm(-1)) and infrared (4000-200 cm(-1)) spectra of 3-chloro, 4-chloro and 5-chloro-2-methylphenyl isocyanates have been measured. Ab initio and density functional theory calculations, at the levels of RHF/6-311G* and B3LYP/6-311G*, have been performed: energies, optimized geometrical parameters, vibrational frequencies, infrared intensities, Raman activities, depolarization ratios and nuclear displacements are obtained. Potential energy distributions (PEDs) and normal modes, for the spectral data computed at B3LYP/6-311G*, have also been obtained from a force-field calculations. A complete vibrational assignments of the observed spectra have been proposed. The force-field calculations have shown that, several of the normal modes are coupled, as is the case with large molecular systems possessing very low or no symmetry, such as investigated in the present study. Further, the investigation of the internal rotation of the isocyanate, NCO, by B3LYP/6-31G* level of theory has shown that the moiety maintains nearly the same orientation in all the three compounds (approximately 140-145 degrees tilt to the para-position) as in phenyl isocyanate. Two conformers, cis and trans forms, with respect to the substituents, NCO and CH(3), have been determined: the cis form lies above trans form by less than a kilocalorie per mole for each compound.     

Spectral assignments for 1H, 13C and 15N solution and solid-state NMR spectra of s-tetrazine and dihydro-s-tetrazine derivatives

Tetrazine-based organic species are interesting intermediates for organic synthesis and represent a source of new materials bearing specific properties with potential applications in biology and material science. 1H, 13C, 15N NMR measurements carried out in solution and in the solid-state have been used to characterize a series of 3,6-disubstituted 1,2,4,5-tetrazine/dihydrotetrazine new derivatives. Experimental results presented here provide data for the assignment of 15N chemical shifts including new organic small molecules; two polymers having the tetrazine ring in the main chain and several previously published compounds. We report apparently for the first time 15N experimental chemical shift data for tetrazine systems in the solid state.     

Ab initio characterization of XH3 (X = N,P). Part II. Electric, magnetic and spectroscopic properties of ammonia and phosphine

The coupled cluster theory in conjunction with core valence triple and quadruple zeta basis sets has been employed for investigating electric, magnetic and spectroscopic properties of ammonia and phosphine. Namely molecular dipole and quadrupole moments, NMR shielding and spin-rotation constants, as well as spectroscopic properties such as rotational and centrifugal distortion constants as well as harmonic and anharmonic frequencies of NH₃ and PH₃ have been determined at a high level of accuracy. To obtain parameters directly comparable to experiment, vibrational effects have also been taken into account. In addition, the basis set convergence has been investigated for the molecular dipole moment.     

FTIR and FTRaman spectra,assignments, ab initio HF and DFT analysis of 4-nitrotoluene

In this work, the experimental and theoretical study on molecular structure and vibrational spectra of 4-nitrotoluene are studied. The FTIR and FTRaman experimental spectra of the molecule have been recorded in the range of 4000–100 cm^?1. Making use of the recorded data, the complete vibrational assignments are made and analysis of the observed fundamental bands of molecule is carried out. The experimental determinations of vibrational frequencies are compared with those obtained theoretically from ab initio HF and DFT quantum mechanical calculations using HF/6-31G (d, p), B3LYP/6-31++G* (d, p) and B3LYP/6-311++G* (d, p) methods. The differences between the observed and scaled wave number values of most of the fundamentals are very small in B3LYP than HF. The geometries and normal modes of vibrations obtained from ab initio HF and B3LYP calculations are in good agreement with the experimentally observed data. Comparison of the simulated spectra provides important information about the ability of the computational method (B3LYP) to describe the vibrational modes. The vibrations of NO₂ and CH₃ groups coupled with skeletal vibrations are also investigated.