State Rotational Transitions Research Articles

Microwave spectrum, r0 structure, barriers to internal rotation and ab initio calculations of gauche-1,1-difluoropropane

The microwave spectra of four isotopomers, CH 3CH 2 12CHF 2, CH 3CH 2 13CHF 2, CD 3CH 2 12CHF 2 and CD 3CD 2 12CHF 2, of gauche-1,1-difluoropropane, in the ground vibrational state were recorded and assigned. From the measured frequencies for the R and Q branches of the a-, b- and c-type transitions, accurate values for the rotational constants were obtained along with values for Δ J , Δ JK , δ J and Δ K . The dipole moment components were evaluated from the measurements of the Stark effects of the ground state rotational transitions and the determined values are: | μ a|=2.044(3), | μ b|=0.851(5), | μ c|=0.711(35) and | μ tot|=2.326(3) D. Ab initio molecular orbital calculations have been carried out with valence electron correlation by the perturbation method to second-order with various basis sets up to MP2(full)/6-311+G(d,p) from which structural parameters, barriers to internal rotation, and distortion constants have been obtained. The C–H distance of 1.095±0.002 Å for the CHF 2 moiety has been determined from the isolated stretching frequency from the CD 3CD 2 12CHF 2 molecule. The r 0 parameters have been obtained by combining the ab initio predicted values with the microwave rotational constants and these parameters for the heavy atoms are: rC 1–F 5=1.370(5), rC 1–F 6=1.372(5), rC 1–C 2=1.505(3), rC 2–C 3=1.522(3) Å, ∠C 1C 2C 3=112.2(5), ∠C 2C 1F 5=109.8(5), ∠C 2C 1F 6=110.9(5)°. The adjusted r 0 parameters have also been obtained for 2,2-difluoropropane, CH 3CF 2CH 3. The results are compared to those of some similar molecules.

Restricted rotation in six-membered cyclic nitrosamine compounds: an ab initio and DFT study and NBO analysis

Besides the fact that in nitrosamines, considered as a family of potent carcinogenic compounds, the barrier to internal rotation about the N–N bond is a significant conformational factor, it is as well important for understanding the mechanism of their bioactivity and detoxification. In this work, a detailed study of the internal rotation of the NO group about the N–N bond in various six-membered cyclic nitrosamine compounds was performed using ab initio (HF and MP2) and DFT methods with 6-31G** basis set. Between the two resulting rotational transition states (TS1 and TS2), TS1 was found to be more stable than TS2 according to the all used methods. The activation energy barriers obtained by ab initio methods were also in good agreement with the experimental values. In the studied compound series, the ab initio (HF and MP2) calculated internal rotation activation Gibbs free energies through TS1, lies between 80.01 and 96.26 kJ/mol. The results showed as well that the planar structure of the N-atom in ground state becomes pyramidal in transition states. In addition, in order to explore the origin of the internal rotation energy barrier, the natural bond orbital analysis (NBO) revealed, specifically, that the lp(N4)→BD*(N O) donor–acceptor interaction is the most significant interaction in nitrosamine compounds. NBO analysis showed also that the s-character of the N4 lone pair orbital and its occupation number increases as approaching the top of the rotational barrier in these studied compounds.

Scaling factors for the prediction of vibrational spectra. II. The aniline molecule and several derivatives

AbstractThe structure of aniline was studied by semiempirical, ab initio, and density functional methods. Complete geometry optimization of the minimum energy structure and of the transition states for internal rotation and inversion of the amino group was carried out using several levels. The performance of the different methods in calculating and describing the vibrational frequencies of aniline was determined. The normal modes were characterized by the magnitudes and direction of the displacement vectors. Three procedures were used to obtain the scaled frequencies, two of them new, using specific scale factors and scaling equations from the benzene molecule. The errors obtained were compared with those calculated through other standard procedures. A reassignment of several bands was made. A comparison of the cost‐effective method and procedure of scaling was carried out. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Synthesis, Structure, and Dynamics of Molybdenum Imido Alkyne Complexes

The monomeric alkyne complexes (η2-alkyne)Mo(NPh)(o-(Me3SiN)2C6H4) (3) have been synthesized by the displacement of isobutylene from (η2-isobutylene)Mo(NPh)(o-(Me3SiN)2C6H4) (2). The alkyne fragment in these complexes is oriented perpendicular to the MoN bond of the cis imido ligand, as confirmed by an X-ray structural analysis of 3e. The deshielded nature of the chemical shifts of the α-carbons and terminal protons of the alkyne fragments in these complexes strongly suggests the participation of the alkyne π⊥ electrons in the Mo−alkyne interaction. The alkyne fragment in 3 rotates freely about the Mo−alkyne bond, resulting in the fluxional behavior of these complexes at room temperature. An activation barrier of 13.2 kcal/mol for the alkyne rotation was measured using VT NMR spectroscopy. Computational studies using a two-layer ONIOM model, and the B3LYP hybrid functional, provided insight into the Mo−alkyne bonding. The transition state for alkyne rotation has been calculated and is characterized by a parallel orientation of the alkyne fragment to the cis imido ligand. A natural bond orbital (NBO) population analysis reveals that alkyne π⊥ donation to Mo is more extensive in the transition state than in the ground state. Weaker Mo−N(imido) bonds are also observed in the transition state, because π donation from the alkyne ligand competes with imido π donation.

Nucleophilic attack on ketene, 6‐methylene‐2,4‐cyclohexadienylideneketene, 6‐oxo‐2,4‐cyclohexadienylideneketene and 4‐oxo‐2,5‐cyclohexadienylideneketene: an ab initio study

AbstractNucleophilic attack of cyanide anion at the carbonyl carbon was studied computationally at the MP2/6–311+G** level for ketene and at the MP2/6–31+G* and MP2/6–31+G*//HF/6–31+G* levels for all four title compounds. Heats of addition were computed for formation of in‐plane adducts and perpendicular adducts where the terms ‘in‐plane’ and ‘perpendicular’ indicate the direction of nucleophilic attack relative to the ketene plane. Heats of activation were also computed for the two modes of attack. For ketene (1) the in‐plane adduct is favored by 37.6 kcal mol−1 and the in‐plane transition state by 18.8 kcal mol−1. For the 6‐methylene compound (2) the in‐plane adduct is favored by 20.1 kcal mol−1 and the in‐plane transition state by 7.3 kcal mol−1. For the 6‐oxo compound (3) the in‐plane adduct is more stable by 11.2 kcal mol−1 whereas the in‐plane transition state is preferred by 8.8 kcal mol−1. For the 4‐oxo compound (4) the in‐plane adduct is favored by 19.3 kcal mol−1 but its transition state by only 2.8 kcal mol−1. The in‐plane adduct for ketene is the enolate of acetyl cyanide while the perpendicular adduct is a pyramidalized carbanion attached to a cyanocarbonyl group. Calculated NPA charges support this conclusion. The perpendicular adduct is actually the transition state for rotation about the CC bond of the ketene moiety in the in‐plane adduct, and the 37.6 kcal mol−1 enthalpy difference is closely matched by the calculated activation enthalpy for rotation about the CC bond of acetaldehyde enolate: ΔH‡ = 33.6 kcal mol−1. The smaller differences found for in‐plane vs perpendicular adduct formation for the 6‐methylene, 6‐oxo and 4‐oxo compounds are correlated with decreases in bond length alternation within the six‐membered rings which occur upon addition of the nucleophile. Less alternation may be associated with an approach to aromaticity, hence an increase in stability. Both modes of attack lead to reduced bond length alternation, but the amount of alternation is lowered more for the perpendicular adducts than for the in‐plane adducts. Changes in the CO bond lengths are also diagnostic. For all four ketenes the ketenyl CO length in the in‐plane adducts is greater than in the perpendicular adducts reflecting more single bond (enolate) character in the former and more double bond (carbonyl) character in the latter. On the other hand the lengths of the 6‐oxo and 4‐oxo CO bonds in their in‐plane adducts are smaller than in the perpendicular adducts, consistent with increased conjugation with the oxo substituent in the latter. The greater NPA charge on the 6‐oxo and 4‐oxo oxygens of the perpendicular adducts supports this conclusion. Calculated enthalpies of activation are in the order ketene > 6‐methyleneketene > 6‐oxoketene, in qualitative agreement with experimentally determined rates of hydration. Copyright © 2004 John Wiley & Sons, Ltd.

Ammonium cyanate: a DFT study of crystal structure, rotational barriers and vibrational spectrum

The crystal structure, rotational barriers and vibrational spectrum of ammonium cyanate have been studied by DFT calculations. The results show that, in the most stable structure, the ammonium ion is oriented such that each N—H bond points towards the N atoms of a cyanate anion giving rise to N—H···N hydrogen bonding, rather than N—H···O hydrogen bonding. The N—C and C—O bond lengths suggest that the structure of the anion in the crystal is best described as −N=C=O. These structural features are in agreement with recent results from neutron diffraction. At the transition state for rotation of the ammonium cation about an N—H bond, the cation is displaced and distorted from its equilibrium configuration. The barrier to the rotation of the ammonium cation about the 4 axis is found to be larger than the minimum barrier to rotation about an N—H bond, suggesting that the latter is the preferred rotational mode.

Dynamics with a Rotational Transition State

Rotationally inelastic collisions between hydrogen molecules and polar molecules, especially water, are of great importance in assessing the thermodynamic properties of interstellar gases. We show here on a simple model that momentum exchange is efficiently mediated by a relative transition state. It acts as a threshold between an inner region and an outer region around the polar molecule. The geometric properties of classical dynamics at threshold are discussed and a capture type of theory is described.

Facile ketene-ketene and ketene-ketenimine rearrangements: a study of the 1,3-migration of alpha-substituents interconverting alpha-imidoylketenes and alpha-oxoketenimines, a pseudopericyclic reaction.

Theoretical calculations (B3LYP/6-311+G(3df,2p)//B3LYP/6-31G) of the 1,3 migration of NR(2) transforming alpha-oxoketenimines 1 to alpha-imidoylketenes 3 and vice versa indicate that this process is a pseudo-pericyclic reaction with a low activation energy (NH(2) 97 kJ mol(-1), N(CH3)(2) 62 kJ mol(-1)). The oxoketenimines were found to be more stable (by 18-35 kJ mol(-1)) which is in line with experimental observations. The hindered amine rotation in the amide and amidine moieties adjacent to the cumulenes are important in the migration of the NR(2) group, as one of the rotation transition states is close to the 1,3 migration pathway. This gives an interesting potential energy surface with a valley-ridge inflection (VRI) between the orthogonal hindered amine rotation and 1,3 migration transition states. The imidoylketene may also undergo ring closure to an azetinone 5; however, this is metastable, and under the conditions that allow the 1,3-migration, the oxoketenimine 1 will be favored. The imine NH E/Z-interconversion of the ketenimine group takes place by inversion and has a low activation barrier ( approximately 40 kJ mol(-1)). In all the amidines examined the E/Z-interconversion of the imine function was predicted to be by rotation with a high barrier (>80 kJ mol(-1)), in contrast to all other reported imine E/Z-interconversions which are by inversion.

Ab initio conformational analysis of fluoromethyl formate and difluoromethyl formate

Abstract We employed ab initio quantum mechanical techniques, such as HF, MP2, and QCISD methods, to explore the conformational spaces of fluoromethyl formate and difluoromethyl formate, HC(O)OCH 3 − n F n ( n =1,2). Three potential energy minima, two with the syn and one with the anti configuration, were located for fluoromethyl formate, while four potential energy minima, two with the syn and the other two with the anti configuration, were located for difluoromethyl formate. In addition, we characterized several rotational transition states for the rotation of the C–O single bond adjacent to the carbonyl group or that adjacent to the fluoro-substituted methyl group. The results of the energy evaluations of the rotational stationary points suggest that the syn / anti energy difference, as well as the barrier height to the syn / anti interconversion, tends to decrease monotonously on going from methyl formate to trifluoromethyl formate, via fluoromethyl formate and difluoromethyl formate.

Interferometric observations of FeO towards Sagittarius B2

We have used the Nobeyama Millimeter Array (NMA) to carry out aperture synthesis observations of the J = 5−4 ground state rotational transition of FeO molecule at 153.135273 GHz towards the galactic center H region Sagittarius B2 Main (Sgr B2 M). We confirm the detection of this line in absorption with the IRAM 30-m telescope by Walmsley et al. (2002). Due to the higher angular resolution (6. �� 5 × 3. �� 0) of our NMA observations, we were able to show that the absorption has a broader line width and a deeper apparent optical depth toward the central 9. �� 2 × 8. �� 0 area around the ultra-compact (UC) H regions. This suggests a higher column density of FeO towards the UCH regions associated with Sgr B2 M than along adjacent lines of sight sampled with the IRAM 30-m telescope. Our results will be a crucial step toward understanding not only the chemistry of iron-bearing species in the interstellar medium, but also the degree of depletion of heavy elements.

Gas-phase energy differences between the Z and E rotamers and the rotational barrier heights of methyl formate and trifluromethyl formate: an ab initio study

Ab initio energy evaluations on the Z and E rotamers, as well as on the rotational transition states connecting them, were carried out for methyl formate and trifluoromethyl formate. The estimates for the energy differences at the basis set limit suggested that the Z/E energy difference and the rotational barrier height are both significantly smaller for trifluoromethyl formate than for methyl formate. Our best estimate for the enthalpy difference at 298 K between the Z and E rotamers for methyl formate is 5.21 kcal/mol, which is slightly larger than the upper edge of uncertainty of previously reported experimental value.

Rotational transition states: relative equilibrium points in inelastic molecularcollisions

Transition states (TSs) are a key ingredient in the understanding of many chemicalreactions. We present here a generalization of TS theory towards rotational excitationin molecular collisions, in a multi-dimensional classical Hamiltonian framework.The treatment is based on relative equilibrium, where the two colliding moleculesbehave as a single rotating solid object. We illustrate the theory with the help of asimple, yet meaningful, model potential for collisions between H2O andH2,which is of great importance in the astrophysical context. We show that it is theoccurrence of a rotational TS that opens up the possibility of significant angularmomentum transfer.

Accurate scaling of the vibrational spectra of aniline and several derivatives

The structure of aniline was studied by quantum chemical methods. Complete geometry optimization of the minimum energy structure and of the transition states for internal rotation and inversion of the amino group was carried out using several levels. The performance of the different methods in calculating and describing the vibrational wavenumbers of aniline was determined. The normal modes were characterized by the magnitude and direction of the displacement vectors. Three procedures were used to obtain the scaled wavenumbers, two of them new, using specific scale factors and scaling equations from benzene molecule. The errors obtained were compared with those calculated through other standard procedures. A reassignment of several bands was made. A comparison of the cost/effective method and procedure of scaling was carried out. Specific scale factors and scale equations were determined for the amino group to be used in large aromatic amines.

Sub-Doppler Saturation Spectroscopy of HCN up to 1 THz and Detection of J = 3 —> 2 (4—> 3) Emission from TMC1

Very high-resolution ( ∼ 30 kHz) and very precise (±2 kHz) saturation dip and crossover dip measurements are reported for HCN. Nine consecutive rotational transitions of the vibrational ground state were recorded, covering the rotational spectrum up to the J = 11 ← 10 transition at 975 GHz. Commencing the saturation dip measurements with the J = 3 ← 2 transition located at 265 886.4 MHz, all rotational transitions were measured up to J = 11 ← 10 (ΔF = 1), positioned at a center frequency of 974 487.2 MHz. It has become possible to resolve the hyperfine structure of every rotational transition to varying degrees. Transitions obeying the selection rules ΔJ = 1, ΔF = 0 are have been resolved, those obeying the selection rules J = 1, F = 1 are only resolved for transitions lower than the J = 6 ← 5 transition. These new experimental saturation dip data, together with the molecular beam maser emission data of the J = 1 → 0 and J = 2 → 1 transitions reported by De Lucia and Gordy, (Phys. Rev. 187, 58 (1969)), and the recent terahertz measurements performed in this laboratory up to J = 22-21 at 1.946 THz (Maiwald et al., J. Mol. Spectrosc. 202, 166 (2000)), were subjected to a least squares analysis which yielded a highly precise set of molecular constants for the ground state of HCN: B = 44 315.974 970 (156) MHz, D = 0.087 216 35 (169) MHz, H = 0.086 96 (242) Hz; eQq = -4.709 03 (162) MHz, eQqJ = 0.244 (88) Hz, CN = 10.09 (38) kHz, CNJ = -0.0143 (86) mHz. Two constants, the hydrogen spin-rotation, CH = -4.35 (5) kHz, and the spin-spin interaction between the proton and nitrogen nucleus, SNH = 0.154 (3) kHz, can not be determined from the saturation dip measurements and have been taken from Ebenstein and Muenter, J. Chem. Phys. 80, 3989 (1984). There also a value for the ground state permanent electric dipole moment (in Debye’s) is given, which we quote for completeness: 〈μ〉000 = 2.985 188 (3) D. We also report the discovery of the J = 3 → 2 and J = 4 → 3 ground state rotational transitions of HCN in the dark, cold molecular cloud TMC1 by using the KOSMA 3m-Submillimeter Telescope located in the central Swiss Alps. For the J = 3 → 2 transition the hyperfine splitting has partly been resolved. The intensities of the hyperfine components are anomalous, and they are not in thermodynamic equilibrium.

Microsolvation of the ammonia cation in argon: I. Ab initio and density functional calculations of NH 3+–Ar n ( n=0–5)

The intermolecular interaction and microsolvation process of the ammonia cation (NH 3 +, ammoniumyl) in its 2 A 2 ″ ground electronic state with up to five Ar ligands are investigated by quantum chemical ab initio and density functional calculations at the unrestricted HF, MP2, and B3LVYP levels of theory using a basis set of aug-cc-pVTZ quality. The global minimum of the intermolecular potential energy surface (PES) of the NH 3 +–Ar dimer calculated at the MP2 level corresponds to a planar proton-bound H 2NH +–Ar structure with C 2v symmetry. The linear N–H–Ar bond is characterized by binding energies of D e =1133 cm −1 and D 0=897 cm −1 and an intermolecular H–Ar separation of R e =2.22 A ̊ . The p-bound structure, in which the Ar ligand is attached to the partially filled 2p z orbital of nitrogen along the C 3 rotation axis, is a slightly less stable local minimum with C 3v symmetry, dissociation energies of D e =866 cm −1 and D 0=672 cm −1 , and a N–Ar separation of R e =2.95 A ̊ . The planar side-bound structure, in which the Ar ligand is attached to the side of the NH 3 + triangle (C 2v symmetry, D e =813 cm −1 , D 0=726 cm −1 , R N– Ar =3.24 A ̊ ) , is a transition state for in-plane internal rotation connecting two equivalent H-bound global minima with a low barrier of V b =320 cm −1 . Analysis of the charge distributions shows that the attraction between Ar and the open-shell NH 3 + ion is dominated almost exclusively by induction forces for all angular orientations. The most stable structures of larger NH 3 +–Ar n clusters ( n=1–5) are highly symmetric and form two distinct subshells within the first solvation shell. The first three Ar ligands form equivalent (nearly) linear proton bonds to the three protons of NH 3 + leading to structures with C 2v and D 3h symmetry. The next two Ar ligands are attached to opposite sides of the 2p z orbital of N leading to geometries with C 3v and D 3h symmetry, respectively. The intermolecular proton bonds significantly destabilize the intramolecular N–H bonds, whereas the p-bonds strengthen them slightly. Noncooperative effects are observed for this prototype solvation of a XH 3 + ion by nonpolar spherical ligands. Comparison between XH 3 +–Ar n (X=C, N, O) and NH k +–Ar n ( k=2–4) reveals the influence of the electron density in the 2p z orbital of XH 3 + and the number of equivalent protons in NH k + on the dimer PES and the microsolvation process.

An ab initio study of conformations and sigmatropic shifts in triazene and its mono-, di-, and trimethyl derivatives

A computational study of triazene, methyltriazene, dimethyltriazene, and trimethyltriazene is presented. A number of different conformers are analyzed and rationalized using hyperconjugation and steric interaction arguments. The transition states for rotation, inversion, and proton and methyl shifts (including water-mediated) are found and the barriers determined.Key words: triazene, ab initio, methyltriazene, 1,3-proton shift.

Methyl rotational barriers in amides and thioamides.

The methyl rotational barriers for a series of N-methyl-substituted amides and thioamides have been calculated at the MP2/6-311+G** level. A comparison of the N-methylformamide and methyl formate barriers indicates that the H [bond] C(Me) [bond] N [bond] H eclipsed torsional arrangement destabilizes an amide by about 0.8 kcal/mol. A comparison of thioamides and amides showed the importance of steric repulsion between the sulfur and a methyl hydrogen in the Z-forms of the thioamides. The C [bond] N bond rotation transition states of the N,N-dimethyl amides have much larger methyl rotational barriers than found in the ground states. They can be attributed to the smaller CH(3)(-)N [bond] CH(3) bond angles in the transition states.

Ground-state molecular stabilization of substituted ethylenes. A theoretical mo ab-initio thermochemical study

The origin of the rotational barrier around the partial CC double bond in substituted ethylenes is discussed with reference to the stabilization of the conformational minimum (GS) and of the rotational transition state (TS). Molecules with different polar character of the double bond were chosen, ranging from ethylene to olefins with strong push–pull character. The enthalpies of hydrogenation, ΔHhydr., and of formation, ΔHf0, of these molecules were employed to obtain the stability of GS; these thermochemical properties were calculated with MO ab-initio theory at HF/6-311G∗∗//HF/6-311G∗∗ and MP2/6-311G∗∗//HF/6-311G∗∗ levels and with CBS-4M model chemistry. The stabilization of TS was derived from the torsional potential for rotation around the CC bond. The lowering of the energy content of GS of substituted ethylenes, referring to ethylene, is accompanied by an even greater stabilization of TS, thus a lowering of the rotational barrier with respect to ethylene is generally found in these molecules.

Experimental and theoretical studies on the thermal decomposition of heterocyclic nitrosimines.

A series of substituted 2-nitrosiminobenzothiazolines (2) were synthesized by the nitrosation of the corresponding 2-iminobenzothiazolines (6). Thermal decomposition of 2a--f and of the seleno analogue 7 in methanol and of 3-methyl-2-nitrosobenzothiazoline (2a) in acetonitrile, 1,4-dioxane, and cyclohexane followed first-order kinetics. The activation parameters for thermal deazetization of 2a were measured in cyclohexane (Delta H(++) = 25.3 +/- 0.5 kcal/mol, Delta S(++) = 1.3 +/- 1.5 eu) and in methanol (Delta H(++) = 22.5 +/- 0.7 kcal/mol, Delta S(++) = -12.9 +/- 2.1 eu). These results indicate a unimolecular decomposition and are consistent with a proposed stepwise mechanism involving cyclization of the nitrosimine followed by loss of N(2). The ground-state conformations of the parent nitrosiminothiazoline (9a) and transition states for rotation around the exocyclic C==N bond, electrocyclic ring closure, and loss of N(2) were calculated using ab initio molecular orbital theory at the MP2/6-31G* level. The calculated gas-phase barrier height for the loss of N(2) from 9a (25.2 kcal/mol, MP4(SDQ, FC)/6-31G*//MP2/6-31G* + ZPE) compares favorably with the experimental barrier for 2a of 25.3 kcal/mol in cyclohexane. The potential energy surface is unusual; the rotational transition state 9a-rot-ts connects directly to the orthogonal transition state for ring-closure 9aTS. The decoupling of rotational and pseudopericyclic bond-forming transition states is contrasted with the single pericyclic transition state (15TS) for the electrocyclic ring-opening of oxetene (15) to acrolein (16). For comparison, the calculated homolytic strength of the N--NO bond is 40.0 kcal/mol (MP4(SDQ, FC)/6-31G*//MP2/6-31G* + ZPE).

Exocyclic push–pull conjugated compounds. Part 4: rotational barriers in poorly polarized push–pull ethylenes

The rotational barrier for substituted ethylenes was calculated with MO ab initio methods, both in the Hartree–Fock (HF) single determinant scheme and with multiconfigurational self-consistent field (MCSCF) approaches. The HF model affords reliable results only when applied to molecules substituted with strong electron donor groups, assigning a push–pull character to the molecule. The MCSCF approach was employed for calculating rotational barriers in poorly polarized ethylenes not directly amenable to the HF methods; however, this method was found hard to handle for low symmetry molecules with substituents interacting with the double bond. A method is proposed based on the interpolation of the energy of the perpendicular conformation from a Fourier truncated function constructed with HF molecular energies calculated for frozen conformations twisted up to 60°. The application of HF theory for studying internal rotation in substituted ethylenes with poorly polarized character is discussed and an upper limit of 35–40kcal/mol can be set up for having reliable barriers from the calculated energy of the rotational transition state at this level of theory.