Inversion barriers in NH2X,PH2X,NHXY, and PHXY species

Ab initio calculations are presented on the inversion barriers in mono-substituted amines and phosphines (NH₂X and PH₂X) and their radical analogs. As expected, using MP3/6-31G^* energies, phosphine barriers are rather larger than those for the corresponding amines, and electronegative substituents increase the barriers. Cyano substitution causes a dramatic decrease in the ammonia barrier. All of the radical nitrogen species studied are predicted to have small or zero barriers, and the radical NH₂O is predicted to be planar. PH₂O also has a very small barrier.     

Silylanions: inversion barriers and NMR chemical shifts

Alpha-substituent effects on inversion barriers and NMR chemical shifts have been studied on a set of silyl anions, [X(3-n)Y(n)Si](-) (X, Y=H, CH(3), and SiH(3)). The MP2/6-31+G* optimized structures show a pattern of increasing inversion barriers with augmenting numbers of methyl substituents. The highest barrier of 48.5 kcalmol(-1) is obtained for the (CH(3))(3)Si(-) ion. The silyl group displays the opposite effect by decreasing the inversion barrier to a minimum of 16.3 kcalmol(-1) in (SiH(3))(3)Si(-). The influence of counterions on these barriers is probed by addition of a lithium or potassium cation. In most cases, a decrease of the energy barriers with respect to the bare anions is observed. The (29)Si NMR chemical shifts calculated at the IGLO-DFT and GIAO-MP2 level of theory are also analyzed in view of the substituents and counterions.     

Asymmetry and Non‐Adiabaticity in Fragmentation of Disulfide Bonds upon Electron Capture

Although it has been generally assumed that electron attachment to disulfide derivatives leads to a systematic and significant activation of the S? S bond, we show, by using [CH₃SSX] (X=CH₃, NH₂, OH, F) derivatives as model compounds, that this is the case only when the X substituents have low electronegativity. Through the use of MP2, QCI and CASPT2 molecular orbital (MO) methods, we elucidate, for the first time, the mechanisms that lead to unimolecular fragmentation of disulfide derivatives after electron attachment. Our theoretical scrutiny indicates that these mechanisms are more intricate than assumed in previous studies. The most stable products, from a thermodynamic viewpoint, correspond to the release of neutral molecules; CH₄, NH₃, H₂O, and HF. However, the barriers to reach these products depend strongly on the electronegativity of the X substituents. Only for very electronegative substituents, such as OH or F, the loss of H₂O or HF is the most favorable process, and likely the only one observed. This is possible because of two concomitant factors, 1) the extra electron for [CH₃SSX]^? (X=OH, F) occupies a σ*(S? X) MO, which favors the cleavage of the S? X bond, and 2) the activation barriers associated with the hydrogen transfer process to produce H₂O and HF are rather low. Only when the substituents are less electronegative (X=H, CH₃, NH₂) the extra electron is located in a σ*(S? S) orbital and the cleavage of the disulfide bridge becomes the most favorable process. The intimate mechanism associated with the S? S bond dissociation process also depends strongly on the nature of the substituent. For X=H or CH₃ the process is strictly adiabatic, while for X=NH₂ it proceeds through a conical intersection ( CI ) associated with the charge reorganization necessary to obtain, from a molecular anion with the extra electron delocalized in a σ*(S? S) antibonding orbital, two fragments with the proper charge localization.     

NR transfer reactivity of azo-compound I of P450. How does the nitrogen substituent tune the reactivity of the species toward C-H and C=C activation?

We studied electronic structures and reactivity patterns of azo-compound I species (RN-Cpd I) by comparison to O-Cpd I of, e.g., cytochrome P450. The study shows that the RN-Cpd I species are capable of C=C aziridination and C-H amidation, in a two-state mechanism similar to that of O-Cpd I. However, unlike O-Cpd I, here the nitrogen substituent (R) exerts a major impact on structure and reactivity. Thus, it is demonstrated that Fe=NR bonds of RN-Cpd I will generally be substantially longer than Fe=O bonds; electron-withdrawing R groups will generate a very long Fe=N bond, whereas electron-releasing R groups should have the opposite effect and hence a shorter Fe=N bond. The R substituent controls also the reactivity of RN-Cpd I toward C=C and C-H bonds by exerting steric and electronic effects. Our analysis shows that an electron-releasing substituent will lower the barriers for both bond activation reactions, since the electronic factor makes the reactions highly exothermic, while an electron-withdrawing one should raise both barriers. The steric bulk of the substituent is predicted to inhibit more strongly the aziridination reactions. It is predicted that electron-releasing substituents with small bulk will create powerful aziridination reagents, whereas electron-withdrawing substituents like MeSO(2) will prefer C-H bond activation with preference that increases with steric bulk. Finally, the study predicts (i) that the reactions of RN-Cpd I will be less stereospecific than those of O-Cpd I and (ii) that aziridination will be more stereoselective than amidation.     

Linear ketenimines. Variable structures of C,C-dicyanoketenimines and C,C-bis-sulfonylketenimines

C,C-dicyanoketenimines 10a-c were generated by flash vacuum thermolysis of ketene N,S-acetals 9a-c or by thermal or photochemical decomposition of alpha-azido-beta-cyanocinnamonitrile 11. In the latter reaction, 3,3-dicyano-2-phenyl-1-azirine 12 is also formed. IR spectroscopy of the keteniminines isolated in Ar matrixes or as neat films, NMR spectroscopy of 10c, and theoretical calculations (B3LYP/6-31G) demonstrate that these ketenimines have variable geometry, being essentially linear along the CCN-R framework in polar media (neat films and solution), but in the gas phase or Ar matrix they are bent, as is usual for ketenimines. Experiments and calculations agree that a single CN substituent as in 13 is not enough to enforce linearity, and sulfonyl groups are less effective that cyano groups in causing linearity. C,C-bis(methylsulfonyl)ketenimines 4-5 and a C-cyano-C-(methylsulfonyl)ketenimine 15 are not linear. The compound p-O2NC6H4N=C=C(COOMe)2 previously reported in the literature is probably somewhat linearized along the CCNR moiety. A computational survey (B3LYP/6-31G) of the inversion barrier at nitrogen indicates that electronegative C-substituents dramatically lower the barrier; this is also true of N-acyl substituents. Increasing polarity causes lower barriers. Although N-alkylbis(methylsulfonyl)ketenimines are not calculated to be linear, the barriers are so low that crystal lattice forces can induce planarity in N-methylbis(methylsulfonyl)ketenimine 3.     

NMR Studies of Restricted Rotation in Aminatophosphorus(1+) Salts

Abstract

A series of aminatophosphorus(1+) salts were synthesized as potential fungicides. Proton NMR of a set of compounds 1 within this series were observed to show selective line broadening of the methylene protons indicating a dynamic process which is slow on the NMR time scale. X-ray crystal structures of two of the compounds in the series showed a relatively planar nitrogen eliminating nitrogen inversion as the source of the observed line broadening. The methylene protons appeared as a sharp doublet (³J_PH = 3.6 Hz) at room temperature when R=OCH₃, a broad singlet when R=OCF₃, and a sharp multiplet consistent with the AB portion of an ABX (X = ³¹P) spin system when R=SCH. Examination of the temperature dependence of the spectra revealed that the observed lineshape and temperature effects were consistent with slow rotation about the N-Ph bond and dependent primarily upon the size of the substituent R. Thus the slow rotation was thought to be due to steric factors and not the influence of electronic effects of the substituent R on the P-N bond. Rotation rates estimated from NMR lineshape analysis and plotted as a function of temperature for 1 when R = SCH₃ and R¹ = Ph gave an calculated energy barrier ΔG₂₉₈ ^? of 17 kcal/mol. Similar studies for a variety of substituents R might be useful as a means of measuring relative steric bulk. At low temperature (ca. -50°C) broadening of the PPh, resonances began to appear indicating a second independent dynamic process thought to be slow rotation about the N-PPh₃ bond on the NMR time scale at that temperature.     

Protonenresonanzuntersuchungen an Aminoboranen—II: Einflüsse von para-Phenylsubstituenten auf die Rotationsbarriere um die N?B-Bindung

The electronic influence of substituents on the free enthalpy of rotation around the N? B bond in aminoboranes was investigated in two series of compounds: (a) (CH₃)₂N?BCl (phenyl-p-X), containing the para-phenyl substituent at the boron atom, and (b) (p-X-phenyl)CH₃N?B(CH₃)₂, containing the para-phenyl substituent at the nitrogen atom of the N? B linkage (X = ? NR₂, ? OCH₃, ? C(CH₃)₃, ? Si(CH₃)₃, ? H, ? F, ? Cl, ? Br, ? I, ? CF₃ and ? NO₂). By comparing the rotational barriers in corresponding compounds of both series, a reverse effect of the substituents could be observed. Electron-withdrawing substituents in the para position of the phenyl ring increase the ΔG_c^≠ if the phenyl group is attached to the boron atom; on the other hand, a lower ΔG_c^≠ is observed if the phenyl ring is bonded to the nitrogen atom of the N? B system. Substitution of the phenyl ring with electron-donating substituents in the paraposition exerts the opposite effect. Within each series of compounds, the differences of ΔG_c^≠ values [δ(ΔG_c^≠) = ΔG_c^≠ (X) ? ΔG_c^≠ (X = H)] between substituted and unsubstituted compounds can be explained in terms of inductive and mesomeric effects of the ring substituents and can be correlated with the Hammett σ constant of each substituent. A comparison of the slopes of the plotted lines shows that the influence of the ring substituents is more pronounced in compounds with N-phenyl-p-X than in those with B-phenyl-p-X.     

Factors Controlling β‐Elimination Reactions in Group 10 Metal Complexes

Trends in reactivity of β‐chloride and β‐hydride elimination reactions involving Group 10 transition‐metal complexes have been computationally explored and analyzed in detail by DFT. These reactions do not require the initial formation of a vacant coordination site; they proceed concertedly without a prior ligand‐dissociation step. Whereas β‐chloride elimination is associated with relatively moderate activation barriers, the high barriers calculated for analogous β‐hydride eliminations suggest that the latter process is unfeasible for this type of compounds. This differential behavior is analyzed within the activation strain model, which provides quantitative insight into the physical factors controlling these β‐elimination reactions. The effects of the nature of the Group 10 transition metal (Ni, Pd, Pt), as well as the substituents attached to the β‐eliminating fragment (R₂C?CR₂X; R, X=H, Cl) on the transformation have also been considered and are rationalized herein.     

Ketene-ketenimine rearrangements in the gas phase and in polar media. 1,3-Migration intermediates and sequential transition states

[reaction: see text] Calculations of the activation barrier for the 1,3-shifts of substituents X in alpha-imidoylketenes 1 (HN=C(X)-CH=C=O), which interconverts them with alpha-oxoketenimines 3 (HN=C=CH-C(X)=O) via a four-membered cyclic transition state TS2 have been performed at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G* level. Substituents with accessible lone pairs have the lowest activation barriers for the 1,3-shift (halogens, OR, NR2). The corresponding activation barriers for the alpha-oxoketene-alpha-oxoketene rearrangement of 4 via TS5 are generally lower by 1-30 kJ/mol. A polar medium (acetonitrile, epsilon = 36.64) was simulated using the polarizable continuum (PCM) solvation model. The effect of the solvent field is a reduction of the activation barrier by an average of 12 kJ/mol. In the cases of 1,3-shifts of amino and dimethylamino groups, the stabilization of the transition state TS2 in a solvent field is so large that it becomes an intermediate, Int2, flanked by transition states (TS2' and TS2') that are due primarily to internal rotation of the amine functions, and secondarily to the 1,3-bonding interaction. In the case of the alpha-oxoketene-alpha-oxoketene rearrangement of 4, there is a corresponding intermediate Int5 for the 1,3-amine shift already in the gas phase.     

The periodic table and the intrinsic barrier in s(n)2 reactions

The identity S(N)2 reactions on nitrogen (see eq 3) with nucleophiles having the general structure H(n)()X(-) where X belongs to the group of nonmetallic elements which do not border the line separating them from the metallic elements (X = F, Cl, Br, I, O, S, Se, N, P, and C) were studied at the G2+ level. The results show that, similarly to the previously observed phenomenon for S(N)2 reaction on carbon (J. Am. Chem. Soc. 1999, 121, 7724), the Periodic Table, through the valence of the element X, controls the intrinsic barrier for the reaction. The average intrinsic barriers obtained for nitrogen substrates were 20, 27, 39, and 57 kcal/mol for the mono-, di-, tri-, and tetravalent X's, respectively. It is also concluded that the intrinsic barriers are similar for N- and C-based substrates and dimethyl substitution on both raises the intrinsic barrier by ca. 10 kcal/mol.     

Inversion of tetrahedral configuration of bis-{4-aminomethylenepyrazole-5-thion(seleno)ato} nickel(II) complexes

Novel bis-chelate Ni(II) complexes with a series of 1,3-disubstituted 4-aminomethylenepyrazole-5-thion(seleno) derivatives have been prepared. An inversion of the tetrahedral configuration at the metal centre in these complexes proceeds in solution via the intramolecular diagonal twist rearrangement on the NMR time scale which was followed by coalescence studies on several pairs of diastereotopic protons. The activation barriers are: ΔG²(T_c) = 41–74 kJ mol^?1, _c = 218–373 K. The bulky substituents R₃ at the aldimine nitrogen give rise to significant increases in the energy barriers to enantiomerization of tetrahedral complexes owing to the steric hindrance involving an intermediate square planar form.     

La basicité de l'atome d'azote dans quelques iminophosphanes (PNR) et aminophosphanes (PNR2). Etude en spectrographie infrarouge

Using i.r. spectroscopy the lone pair basicity of a nitrogen atom on two iminophosphanes (PN-) and six aminophosphanes (PN-) has been estimated. This was done measuring the decreasing Δν_XH between ν_XH absorption of one XH vibrator (HO- or HN) bonded to a nitrogen atom by hydrogen bonding (PN … HX; PN … HX) and the free absorption ν_XH of the same vibrator. For identical substituents on the phosphorus atom, the basicity of nitrogen in iminophosphanes is more important than in aminophosphanes.     

Effect of substituents and hydrogen bonding on barrier heights in dehydration reactions of carbon and silicon geminal diols

Activation barrier heights for the dehydration reaction of geminal carbinols and silanediols R'R″X(OH)(2) (X = C, Si) were estimated at the B3LYP and MP2 levels of theory employing Dunning's correlation-consistent triple-zeta basis sets. It was shown that the barrier height for carbon derivatives steadily decreases upon substitution by R groups, usually termed as electron-donating, such as alkyl and amino groups. Substitution by electron-withdrawing groups leads, however, only to small changes in barrier heights compared to that of methanediol. A similar tendency was also found for silicon derivatives, but high activation barriers of this reaction remain even for amino group substituted silanediols. Introduction of additional water molecules into the reactive space of carbinol dehydration drastically reduces barrier heights and brings the transition state energy for methanediol close to the experimental value. The difference between dehydration barrier heights for both methanediol and carbinols with electron-rich substituents becomes well-defined for dimeric species. The higher acidity of the hydroxyl group protons in molecules containing halogens and C==O groups brings about a noticeable growth in the dehydration barrier heights of these compounds. This difference in barrier heights for oligomeric species may be the reason for the stability of carbinols with electron-rich substituents.     

Dramatic substituent effects on the mechanisms of nucleophilic attack on Se—S bridges

The reactions of XSeSX, XSeSY, and YSeSX (X, Y = CH₃, NH₂, OH, F) with F^? and CN^? nucleophiles have been investigated by means of B3PW91/6‐311+G(2df,p) and G4 calculations. In systems where the two substituents are not identical (XSeSY), the more stable of the two possible isomers corresponds to those in which the most electronegative substituent is attached to Se. Nucleophilic attack takes place at Se, independent of the nature of the nucleophile, with the only exception being XSeSF (X = CH₃, NH₂, OH), in which case the attack occurs at S. In agreement with recent results for disulfide and diselenide linkages, the mechanisms leading to Se—S bond cleavage are not always the more favorable ones because for highly electronegative substituents the most favorable process is fission of the chalcogen‐substituent bond. These dissimilarities in the observed reactivity pattern as a function of the electronegativity of the substituents are due to the fact that the σ‐type Se—S antibonding orbital, which for low‐electronegative substituents is the lowest unnoccupied molecular orbital (LUMO), becomes strongly destabilized when the electronegativity of the substituent increases, and is replaced by an antibonding π‐type Se‐X (or S‐X) orbital. In contrast, however, with what has been found for disulfide and diselenide derivatives, the observed reactivity does not change with the nature of the nucleophile. The activation strain model provides interesting insight into these processes, showing that in most cases the activation barriers are the consequence of subtle differences in the strain or in the interaction energies. © 2013 Wiley Periodicals, Inc.     

N‐arylamides and N‐arylcarbamates N?CO internal rotation barrier study by molecular modeling

Amides and carbamates present an energetic barrier associated to N? C(O) bond rotation, which determines two different equilibrium geometries. In this work, the conformational equilibrium of formanilide, acetanilide, methyl and t‐butyl phenylcarbamates, and their N‐methylderivatives was studied by AM1 and B3LYP/6‐31G(d,p) calculations. The effect of aryl p‐substituents (MeO, Me, Cl, Br, CN, and NO₂) was also studied. Amide barriers were found by DFT calculation between 12 and 21 kcal/mol. Carbamates, on the other hand, showed barriers between 11 and 15 kcal/mol. AM1 underestimates the energetic barriers and provides values around half those obtained by B3LYP/6‐31G(d,p) calculations. Electron withdrawing substituents on aryl group decrease the barrier. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Heteroatom Tuning of Bimolecular Criegee Reactions and Its Implications

High‐level quantum‐chemical calculations have been performed to understand the key reactivity determinants of bimolecular reactions of Criegee intermediates and H₂X (X=O, S, Se, and Te). Criegee intermediates are implicated as key intermediates in atmospheric, synthetic organic, and enzymatic chemistry. Generally, it is believed that the nature and location of substituents at the carbon of the Criegee intermediate play a key role in determing the reactivity. However, the present work suggests that it is not only the substitution of the Criegee intermediate, but the nature of the heteroatom in H₂X that also plays a crucial role in determining the reactivity of the interaction between the Criegee intermediate and H₂X. The barriers for the reactions of Criegee intermediates and H₂X satisfy an inverse correlation with the bond strength of X−H in H₂X, and a direct correlation with the first pK_a of H₂X. This heteroatom tuning causes a substantial barrier lowering of 8–11 kcal mol⁻¹ in the Criegee reaction barrier in going from H₂O to H₂Te. An important implication of these results is that the reaction of the Criegee intermediate and H₂S could be a source of thioaldehydes, which are important in plantery atmospheres and synthetic organic chemistry. By performing the reaction of Criegee intermediates and H₂S under water or acid catalysis, thioladehydes could be detected in a hydrogen‐bonded complexed state, which is significantly more stable than their uncomplexed form. As a result, simpler aliphatic thioaldehydes could be selectively synthesized in the laboratory, which, otherwise, has been a significant synthetic challenge because of their ability to oligomerize.     

C−X Bond Activation by Palladium: Steric Shielding versus Steric Attraction

The C−X bond activation (X = H, C) of a series of substituted C(n°)−H and C(n°)−C(m°) bonds with C(n°) and C(m°) = H₃C− (methyl, 0°), CH₃H₂C− (primary, 1°), (CH₃)₂HC− (secondary, 2°), (CH₃)₃C− (tertiary, 3°) by palladium were investigated using relativistic dispersion-corrected density functional theory at ZORA-BLYP-D3(BJ)/TZ2P. The effect of the stepwise introduction of substituents was pinpointed at the C−X bond on the bond activation process. The C(n°)−X bonds become substantially weaker going from C(0°)−X, to C(1°)−X, to C(2°)−X, to C(3°)−X because of the increasing steric repulsion between the C(n°)- and X-group. Interestingly, this often does not lead to a lower barrier for the C(n°)−X bond activation. The C−H activation barrier, for example, decreases from C(0°)−X, to C(1°)−X, to C(2°)−X and then increases again for the very crowded C(3°)−X bond. For the more congested C−C bond, in contrast, the activation barrier always increases as the degree of substitution is increased. Our activation strain and matching energy decomposition analyses reveal that these differences in C−H and C−C bond activation can be traced back to the opposing interplay between steric repulsion across the C−X bond versus that between the catalyst and substrate.     

Zur synthese und struktur von pentacarbonylmetall(0)-Komplexen intramolekular basenstabilisierter zinn(III)-verbindungen

The synthesis and structure of pentacarbonylmetal(0) complexes of the types (C0)₅MSn(XCH₂CH₂)₂E (M = Cr, Mo, W; X = O, S; E = NR, PPh, O, S) and (CO)₅ WSn(SCH₂CH₂)₂E · py (E = NMe, O, S) are reported.¹¹⁹Sn and ¹³C NMR studies show the compounds with X = O and E = NR to be dimeric whereas the derivatives with X = S have been found to be monomeric. The π-acceptor behaviour of the ligands Sn(XCH₂CH₂)₂E is comparable to that of the phosphanes. The barriers to internal rotations about the nitrogencarbon bonds have been determined for the NBu^t derivatives.     

Effect of Ligand Field Strength on the Spin Crossover Behaviour in 5‐X‐SalEen (X=Me,Br and OMe) Based Fe(III) Complexes

The reaction of Fe(NCS)₃ prepared in situ in MeOH with 5‐X‐SalEen ligands (5‐X‐SalEen=condensation product of 5‐substituted salicylaldehyde and N‐ethylethylenediamine) provided three Fe(III) complexes, [Fe(5‐X‐SalEen)₂]NCS; X=Me ( 1 ), X=Br ( 2 ), X=OMe ( 3 ). All the complexes reveal similar structural features but a very different magnetic profile. Complex 1 shows a gradual spin crossover while complexes 2 and 3 show a sharp spin transition. The T_1/2 for complex 2 is 237 K while for complex 3 it is much higher with a value of 361 K. The spin transition temperature is shifted towards higher temperature with increasing electron‐donation ability of the ligand substituents. This experimental observation has been rationalized with DFT calculations. UV‐Vis and cyclic voltammetry studies support the fact that the electron density on the ligand increases from Me to Br to OMe substituents. To understand the change in spin states, temperature‐dependent EPR spectra have been recorded. The spin state equilibrium in the liquid state has been probed with Evans NMR spectroscopic method, and thermodynamic parameters have been evaluated for all complexes.     

First‐principles study of molecular hydrogen dissociation on doped Al12X (X = B,Al, C,Si, P,Mg, and Ca) clusters

Inspired by the concept of superatom via substitutionally doping an Al₁₃ magic cluster, we investigated the H₂ molecule dissociation on the doped icosahedral Al₁₂X (X = B, Al, C, Si, P, Mg, and Ca) clusters by means of density functional theory. The computed reaction energies and activation barriers show that the concept of superatom is still valid for the catalysis behavior of doped metal clusters. The hydrogen dissociation behavior on metal clusters characterized by the activation barrier and reaction energy can be tuned by controllable doping. Thus, doped Al₁₂X clusters might serve as highly efficient and low‐cost catalysts for hydrogen dissociation. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009