Stereochemical nonrigidity of homophenalenyl cations

A strong temperature dependence was observed of chemical shifts in the ¹³C NMR spectra of 3b,4-exo-4a,5-tetramethyl-3b,4,4a,5-tetrahydro-4H-cyclopropa[a]phenalen-5-yl cation and its endo-epimer revealing their stereochemical nonrigidity.     

Structural nonrigidity of 9,9,10-trimethyland 4,5,9,9,10-pentamethylphenanthrenylium ions

DFT quantum-chemical calculations predicted structural nonrigidity of 9,9,10-trimethyl- and 4,5,9,9,10-pentamethylphenanthrenylium ions. The cross section of the potential energy surface at the nonrigidity coordinate for the first of these ions is symmetric, and for the second, asymmetric. These results were confirmed experimentally: the ¹³C NMR spectra of 4,5,9,9,10-pentamethylphenanthrenylium showed considerable temperature dependence of chemical shifts which is not typical of 9,9,10-trimethylphenanthrenylium.     

Oxidation of methylamine

A detailed chemical kinetic model for oxidation of methylamine has been developed, based on theoretical work and a critical evaluation of data from the literature. The rate coefficients for the reactions of CHNH + O CHNH / CHNH + HO, CHNH + H CH + NH, CHNH CHNH, and CHNH + O CHNH + HO were calculated from ab initio theory. The mechanism was validated against experimental results from batch reactors, flow reactors, shock tubes, and premixed flames. The model predicts satisfactorily explosion limits for CHNH and its oxidation in a flow reactor. However, oxidation in the presence of nitric oxide, which strongly promotes reaction at lower temperatures, is only described qualitatively. Furthermore, calculated flame speeds are higher than reported experimental values; the model does not capture the inhibiting effect of the NH group in CHNH compared to CH. More work is desirable to confirm the products of the CHNH + NO reaction and to look into possible pathways to NH in methylamine oxidation.     

NMR investigation of stereochemical nonrigidity of five-coordinate phosphorus fluorides

Phenomena associated with stereochemical nonrigidity of five-coordinate phosphorus fluorides (Berry pseudorotation and restricted rotation of different functional groups about the bond with the central phosphorus atom) have been analyzed. The main cause of restricted rotation isp d bonding of unsharedp-electron pairs of substituents with vacantd-orbitals of the phosphorus. Additional stabilization of the molecule occurs when it is possible to form an H...F hydrogen bond between the liquid and an axial fluorine atom in the phosphorane. The relationship between pseudorotation and restricted rotation is considered.A. N. Nesmeyanov Institute of Organoelemental Compounds, Russian Academy of Sciences, 117813 Moscow. N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 117977 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 8, pp. 1784–1793, August, 1992.     

Analysis of classical and quantum paths for deprotonation of methylamine by methylamine dehydrogenase.

The hydrogen-transfer reaction catalysed by methylamine dehydrogenase (MADH) with methylamine (MA) as substrate is a good model system for studies of proton tunnelling in enzyme reactions--an area of great current interest--for which atomistic simulations will be vital. Here, we present a detailed analysis of the key deprotonation step of the MADH/MA reaction and compare the results with experimental observations. Moreover, we compare this reaction with the related aromatic amine dehydrogenase (AADH) reaction with tryptamine, recently studied by us, and identify possible causes for the differences observed in the measured kinetic isotope effects (KIEs) of the two systems. We have used combined quantum mechanics/molecular mechanics (QM/MM) techniques in molecular dynamics simulations and variational transition state theory with multidimensional tunnelling calculations averaged over an ensemble of paths. The results reveal important mechanistic complexity. We calculate activation barriers and KIEs for the two possible proton transfers identified-to either of the carboxylate oxygen atoms of the catalytic base (Asp428beta)-and analyse the contributions of quantum effects. The activation barriers and tunnelling contributions for the two possible proton transfers are similar and lead to a phenomenological activation free energy of 16.5+/-0.9 kcal mol(-1) for transfer to either oxygen (PM3-CHARMM calculations applying PM3-SRP specific reaction parameters), in good agreement with the experimental value of 14.4 kcal mol(-1). In contrast, for the AADH system, transfer to the equivalent OD1 was found to be preferred. The structures of the enzyme complexes during reaction are analysed in detail. The hydrogen bond of Thr474beta(MADH)/Thr172beta(AADH) to the catalytic carboxylate group and the nonconserved active site residue Tyr471beta(MADH)/Phe169beta(AADH) are identified as important factors in determining the preferred oxygen acceptor. The protein environment has a significant effect on the reaction energetics and hence on tunnelling contributions and KIEs. These environmental effects, and the related clearly different preferences for the two carboxylate oxygen atoms (with different KIEs) in MADH/MA and AADH/tryptamine, are possible causes of the differences observed in the KIEs between these two important enzyme reactions.     

Structural and stereochemical nonrigidity of 7-(heptaphenylcycloheptatrienyl) isothiocyanate

By means of DFT B3LYP/6-31G(d, p) calculations of 7-(heptaphenylcycloheptatrienyl) isothiocyanate, the dissociation–recombination mechanism for intramolecular migrations of the isothiocyanato group has been revealed, and the structure of the transition state preceding the formation of a tight ion pair has been found for the first time. According to calculations, the high activation barrier for displacements of the isothiocyanato group ΔE_ZPE^≠ = 21.3 kcal/mol is related to the stable conformation of the molecule with the equatorial–NCS group and the orthogonally located phenyl substituent in the axial position. The rearrangement of the molecule to the form favorable for migrations of the–NCS group involves the inversion of the seven-membered ring accompanied by rotation of the phenyl group.     

Kinetic studies on the cleavage of phthalimide in methylamine buffers

In aqueous methylamine buffers of pH 10.21–11.25 nucleophilic cleavage of ionized (S^–) phthalimide and general base-catalyzed cleavage of nonionized (SH) phthalimide is observed.     

High-resolution spectroscopy of the C-N stretching band of methylamine

The C-N stretching infrared fundamental of CH(3)NH(2) has been investigated by high-resolution laser sideband and Fourier transform synchrotron spectroscopy to explore the energy level structure and to look for possible interactions with high-lying torsional levels of the ground state and other vibrational modes. The spectrum is complicated by two coupled large-amplitude motions in the molecule, the CH(3) torsion and the NH(2) inversion, which lead to rich spectral structure with a wide range of energy level splittings and relative line intensities. Numerous sub-bands have been assigned for K values ranging up to 12 for the stronger a inversion species for the v(t) = 0 torsional state, along with many of the weaker sub-bands of the s species. The C-N stretching sub-state origins have been determined by fitting the upper-state term values to J(J + 1) power-series expansions. For comparison with the ground-state behaviour, both ground and C-N stretch origins have been fitted to a phenomenological Fourier series model that produces an interesting pattern with the differing periodicities of the torsional and inversion energies. The amplitude of the torsional energy oscillation increases substantially for the C-N stretch, while the amplitude of the inversion energy oscillation is relatively unchanged. Independent inertial scale factors ρ were fitted for the torsion and the inversion and differ significantly in the upper state. The C-N stretching vibrational energy is determined to be 1044.817 cm(-1), while the effective upper state B-value is 0.7318 cm(-1). Several anharmonic resonances with v(t) = 4 ground-state levels have been observed and partially characterized. A variety of J-localized level-crossing resonances have also been seen, five of which display forbidden transitions arising from intensity borrowing that allow determination of the interaction coupling constants.     

Hemilability and nonrigidity in metal complexes of bidentate P,PS donor ligands

Hemilability and nonrigidity in a series of mixed P,PS donor ligands has been studied in the complexes [Pd(P,PS)Cl₂], [Pd(η³-C₃H₅)(P,PS)][SbF₆], and [Rh(cod)(P,PS)][SbF₆] (P,PS = Ph₂P-Q-P(S)Ph₂). The effect of bite angle, the rigidity of the ligand backbone, and the role of the ancillary ligands are discussed.     

Ion/molecule reactions of ammonia and methylamine with chloro- and nitrobenzene. A comparison of chemical ionization and ion cyclotron resonance experiments

It is shown by ion cyclotron resonance measurements that ion/molecule reactions, leading to substitution or reduction product ions from chloro- and nitrobenzene with the title amines, are those between the molecular ions [RNH₂]⁺ or [C₆H₅X]⁺˙ and their respective counterparts C₆H₅X or RNH₂. The protonated reagent gas ions [RNH₃]⁺ are not involved in these reactions. In the case of nitrobenzene, adduct ions [C₆H₅NO₂·RNH₃]⁺ do not decompose within the time scale of the measurements. The results obtained are compared with those found under chemical ionization conditions.     

Molecular dynamics study of the active site of methylamine dehydrogenase

We have obtained AMBER94 force-field parameters for the TTQ cofactor of the enzyme methylamine dehydrogenase (MADH). This enzyme catalyzes the oxidation of methylamine to produce formaldehyde and ammonia. In the rate-determining step of the catalyzed reaction, a proton is transferred from the methyl group of the substrate to residue Asp76. We used the new parameters to perform molecular dynamics simulations of MADH in order to characterize the dynamics of the active site prior to the proton-transfer step. We found that only one of the oxygen atoms of Asp76 can act as an acceptor of the proton. The other oxygen interacts with Thr122 via a strong hydrogen bond. In contrast, because of the rotation the methyl group of the substrate, the three methyl hydrogen atoms are alternately in position to be transferred. The distance that the proton has to travel presents a broad distribution with a peak between 1.0 and 1.1 A and reaches values as short as 0.8 A. The fluctuation of the distance between the donor and the acceptor has the largest frequency component at 50 cm(-1), but the spectrum presents a rich structure between 10 and 400 cm(-1). The more important peaks appear below 250 cm(-1).     

Reaction of a phosphorus-containing N-trifluoro-methylfluoroformamidine with methylamine

Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, p. 2868, December, 1989.     

An intraline of conical intersections for methylamine

In this article are considered the conical intersections (ci's) related to the N-H bond in the methylamine, CH(3)NH(2), molecule. The novel feature that was revealed is that the two lowest states 1A(') and 1A(") are coupled by a line of cis located in HC-NHH plane-a line that is formed by moving a single hydrogen on that plane while fixing the (six) other atoms. The validity of this line was proven first by studying the singularities of the (angular) nonadiabatic coupling terms and then by revealing the degeneracy points formed by the two interacting adiabatic potential energy surfaces (PESs). A theoretical analysis indicated that the line has to be a finite closed line. We also calculated the Berry phase for a contour that surrounds this line and found it to be 3.127 rad, namely, a value reasonably close to pi. The existence of such lines of cis-instead of isolated cis (as exhibited by other n-atomic (n>3) molecules such as HNCO or C(2)H(2))-may enhance significantly the transition rate from an upper adiabatic state to a lower one. There are also numerical advantages in such situations, that is, if such a line is properly placed in that plane (like in the present case) the wave-packet treatment of the nuclei can be carried out employing a single diabatic PES instead of having to consider two coupled PESs.     

Paper chromatography of metal ions in solvents containing methylamine

Summary The effect of methylamine and methylamine hydrochloride has been systematically investigated in the paper chromatography of various metal ions. It has been found that methylamine hydrochloride like EDTA helps in preventing tailing. A new separation of Fe, Mn, Ni, and Co has been developed. The R_f-values are 0.97, 0.49, 0.08, 0.79 respectively and the solvent system used is ethylacetoacetate/methylamine hydrochloride/conc. hydrochloric acid (2027). The values are equally spaced, the spots are compact and the separation is probably the best yet recorded.

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Zusammenfassung Es wird über eine systematische Untersuchung des Einflusses von Methylamin und Methylaminhydrochlorid bei der Papierchromatographie verschiedener Metallionen berichtet. Dabei ergab sich, da\ Methylaminhydrochlorid Ähnlich wie ÄDTA zur Verhinderung der Schwanzbildung benutzt werden kann. Zur Trennung von Fe, Mn, Ni und Co wird ein Lösungsmittel aus Acetessigester, Methylaminhydrochlorid und konz. SalzsÄure (2027) empfohlen (R_f-Werte: 0,97, 0,49, 0,08 bzw. 0,79), mit dem sehr gute Ergebnisse erzielt wurden.