CN Bond Lengths Research Articles

Structural investigation of a hindered aromatic amine and its derivatives

Abstract X-ray diffraction structural analysis of N-phenylpiperidine having two isopropyl groups in the ortho (1) and a nitro group in the meta (2) positions, has allowed the confirmation of the findings from molecular mechanics calculations on the simple derivative N-(2,6-diisopropylphenyl)piperidine (1) not bearing the nitro group, that the amino function is heavily twisted about the ipso-CN bond. In addition, there are a number of surprising features like the quasi-planarity of the location of the nitrogen atom and the three carbons attached to it, and the flattened nature of the isopropyl group. The hydrochloride (3) of 1 exhibits a peculiar stoichiometry: a hydronium ion coordinates three chloride ions; two of them also establish polar interactions with two NH groups. A great flattening about the N atoms is observed even in this case. The picrate (4) of 1 has the structure of an ammonium salt. Observed CN bond lengths both for the amino and nitro groups, which are to be found in planar and twisted arrangements, raise basic questions about the accepted and simplified views and correlations of substituent-ring interactions.

Para phenylenediamine radical cation structure studied by resonance Raman and molecular orbital methodsa)

The vibrational and electronic structures of p-phenylenediamine radical cation have been studied using time-resolved resonance Raman spectroscopy and molecular orbital calculations. The Raman spectra in aqueous solution show striking variations in intensity profile when excitation is varied from 340 to 480 nm. Excitation in resonance with the 460–480 nm absorption shows enhancement of only totally symmetric vibrations. Most prominent are the δNH2 scissor mode at 1658 cm−1 and the Wilson modes ν8a (CC stretch) at 1644 cm−1, ν7a (CN stretch) at 1423 cm−1, and ν9a (CH bend) at 1184 cm−1. Also observed are ν1 (ring breathe) at 840 cm−1 and ν6a (CCC bend) at 467 cm−1. With excitation in the 340–410 nm region the ν1 band becomes relatively stronger and an additional band at 1524 cm−1 appears. This latter band, which dominates the spectrum at 360 nm, is assigned to the nontotally symmetric vibration ν8b (CC stretch) having b3g symmetry that gains intensity through vibronic coupling. Raman spectra of ring- and amine-deuterated radicals allow distinction of overtone and combination bands enhanced by Fermi resonance from fundamentals due to δNH2 and ν8a (CC stretch) modes in the 1600–1700 cm−1 region. The theoretical calculations show reasonable agreement with the observed vibrational frequencies and lead to detailed information on the bond structure and normal modes of the radical. The calculated CN bond lengths of 1.33 Å are intermediate between lengths typical of single and double bonds in aromatic amines. The calculations also allow interpretation of the experimental absorption spectrum. The broad medium intensity absorption peaked at 460 nm with a shoulder at 480 nm is assigned to a 2B3u ←2B2g transition, the very weak 360 nm absorption to 2Au ← 2B2g and the very strong ∼325 nm absorption to a higher 2B3u ← 2B2g. Calculations with the simple relation Ik∝ (V′k)2/ωk, which requires evaluation only of vertical excited state gradients, are found to satisfactorily describe the dramatic changes in Raman intensity profile that are observed with different excitation wavelengths. The 460 and 480 nm Raman spectra are governed by scattering from the lowest 2B3u state. Spectra taken in the 410–340 nm range are governed mainly by preresonance scattering from the higher 2B3u state, which is vibronically coupled to the weak 2Au state via the b3g symmetry ν8b (CC stretch) vibration. The relative intensities of totally symmetric modes calculated for excitation into the two 2B3u excited states agree well with those in the two distinct patterns observed experimentally.

Crystallographic studies of intra‐ and inter‐ molecular interactions. Part VI. Crystal and molecular structure of N,N′‐dimethyl‐N′‐phenylsulphonylformamidine. Equalization of CN bond lengths in the amidine fragment as a result of substituent effects due to push–pull

AbstractThe crystal and molecular structure of N,N‐dimethyl‐N′‐phenylsulphonylformamidine is solved by direct methods and refined to R = 0.041 for 935 reflections, Equalization of CN bond lengths in the amidine fragment is discussed in terms of a push—pull effect operating between the N,N‐dimethyl and SO2Ph groups. A significant role of the substituent at the functional carbon on the degree of π‐electron delocalization on the NCN fragment is interpreted by use of the HOSE model.

Application of hyperspherical coordinates to four-atom reactive scattering: H2+CN→H+HCN

We develop the use of Delves’ hyperspherical coordinates to study the reactive scattering of four-atom systems within the collinear approximation. We present quantum mechanical calculations of reaction probabilities for the collinear exothermic reaction H2+CN →H+HCN. We use a potential energy surface which reproduces the essential characteristics of the reaction. The effect of freezing the CN bondlength to its equilibrium value during the reaction is also investigated and is found to be a good approximation. It is found that HCN product vibrational states with the C–H stretch excited are produced preferentially in the reaction.

The electronic structure of CH 2NCH 3, CH 2NNH 2, CH 2NOH and CH 2NF

Ab initio geometry optimisation calculations have been carried out on CH 2NCH 3, CH 2NNH 2, CH 2NOH and CH 2NF using 6-31G and 6-31G** basis sets. Three of the four species possess a plane of symmetry while CH 2NNH 2 prefers a non-planar geometry with the amino N adopting sp 3 hybridisation. The CN bond length remains almost constant at 1.25 Å with an accompanying bond index of 2.2 for all four species. The CNR angle decreases and the related inversion energy at N increases as the R group becomes more electronegative. The charge on nitrogen decreases as the R group changes from CH 3 to F and the energy of protonation at N correspondingly decreases from 230 to 194 kcal mol −1. A slight positive charge is present on the imino carbon atoms and the stabilising energy of hydride ion addition to the imino carbon atom lies between 23.7 kcal mol −1 and 47.4 kcal mol −1.

The electronic structure of trifluoronitrosomethane

A proper description of the geometries and electronic properties of the ground and lowest excited state of CF 3NO requires at least a TCSCF (GVB- pp) wave function. The mixing of CN antibonding character into the SCF configuration is crucial in explaining the unusually long CN bond length. TCSCF calculations correctly reproduce the eclipsed arrangement of the FCNO atoms in the ground state and the staggered conformation in the first excited state. Using a valence double-zeta basis set, the theoretical bond dissociation energy and barriers to internal rotations are in reasonable agreement with experiment. The electronic absorption responsible for the characteristic blue colour of this molecule is identified as the symmetry allowed a 1 A″ ← X 1 A′ excitation. However, the simple TCSCF wave function is not adequate to give the correct transition energy and multireference CI calculations are needed. The theory predicts major structural differences between the ground and excited states. The most noticeable changes are the lengthening of the CF and NO bonds and shortening of the CN bond. The harmonic vibrational frequencies for CF 3NO in the ground and excited state were also calculated by using the TCSCF wave functions. The results are in good agreement with experiment.

ChemInform Abstract: Nonadiabatic Interactions in the Photodissociation of ICN

Photodissociation of ICN in the A continuum has been modeled using classical trajectories assuming that all absorption from the linear ground state is to a single linear diabatic excited state which dissociates to form I*(2P1/2) and CN(2Σ+,v=0). It is also assumed that, in nonlinear excited state configurations, nonadiabatic transitions occur to a bent surface which correlates diabatically to ground state I(2P3/2) and CN(2Σ+,v=0). Empirical potential surfaces with frozen CN bond lengths are employed, while transitions between the surfaces are treated using either the Miller–Meyer classical electron model or a simple diabatic version of the Tully–Preston surface‐hopping model. With the above assumptions, the Miller–Meyer method is found to give much better agreement with the experimental results. Theoretical results obtained with the Miller–Meyer method are compared with recent experimental data on the I*/I branching ratio, the average CN rotational energies, and the product CN rotational distributions as...

Nonadiabatic interactions in the photodissociation of ICN

Photodissociation of ICN in the à continuum has been modeled using classical trajectories assuming that all absorption from the linear ground state is to a single linear diabatic excited state which dissociates to form I*(2P1/2) and CN(2Σ+,v=0). It is also assumed that, in nonlinear excited state configurations, nonadiabatic transitions occur to a bent surface which correlates diabatically to ground state I(2P3/2) and CN(2Σ+,v=0). Empirical potential surfaces with frozen CN bond lengths are employed, while transitions between the surfaces are treated using either the Miller–Meyer classical electron model or a simple diabatic version of the Tully–Preston surface-hopping model. With the above assumptions, the Miller–Meyer method is found to give much better agreement with the experimental results. Theoretical results obtained with the Miller–Meyer method are compared with recent experimental data on the I*/I branching ratio, the average CN rotational energies, and the product CN rotational distributions as a function of photolysis wavelength for λ=248, 266, 280, 290, and 308 nm. Except for the branching ratio at 248 nm, we obtain satisfactory agreement with the experimental results.

Carbon-13/carbon-12 isotope effects on nitrogen-15 chemical shielding in some diamagnetic cyano complexes

Carbon-13/carbon-12 isotope-induced 15N chemical shifts of 0.06 to 0.10 ppm have been observed for the cyanide ion and several transition metal cyanides. In each case, the 15N resonance of the 13C labelled isotopomers is to high field of that in the corresponding carbon-12 species. The sensitivity of the 15N and 13C shielding constants in the cyanide ion to small changes in the CN bond length are evaluated using the theoretical formulation of Jameson: (∂σ(15N)/∂r)e = −872 ± 160 ppm/Å and (∂σ(13C)/∂r)e = −473 ± 90 ppm/Å. Nitrogen-15 chemical shifts in the metal complexes exhibit basically the same behaviour as do the 13C chemical shifts in these same complexes.

An SCF potential energy surface for lithium cyanide

Ab initio SCF calculations are performed for LiCN using a large polarised GTO basis. An analytic fit to the two-dimensional surface (the CN bond length is frozen at 2.186 a 0) is presented.

The effect of salt formation on the molecular structure and charge distribution in imines: the crystal and molecular structures and the ultraviolet, infrared, and the 1H and 13C nuclear magnetic resonance spectra of 2,6-dimethyl-4-(p-nitrophenylimino)pyran and its hydrobromide and methobromide salts

The crystal structures of the title compound (1) and its hydrobromide (2) and methobromide (3) salts have been determined by X-ray crystallography in order to investigate the changes which take place in structure (1) on salt formation. Crystals of both (1) and (2) are monoclinic, with space group P21/c and Z= 4, and having unit cell dimensions respectively a= 12.021(1), b= 8.117(1), c= 13.562(2)A, β= 115.72(1)°, and a= 10.133(2), b= 16.965(4), c= 8.277(2)A, β= 102.55(2)°. Crystals of (3) are triclinic, space group P, and with Z= 2 in a unit cell of dimensions a= 7.929(2), b= 14.480(2), c= 6.549(1)A, α= 90.12(1), β= 98.56(2), γ= 95.37(2)°. Structure (1) was solved by direct methods, and (2) and (3) by the heavy-atom Patterson method; refinements were to R 0.041 for 2 165 independent refelctions in (1), to R 0.047 for 1 855 in (2), and to R 0.032 for 2 754 in (3). There is a systematic increase in CN bond length along the series; all such distances are longer, however, than normal values, as substantiated by vCN frequencies below 1 600 cm–1 in the i.r. spectra. Bond parameters for the pyran ring show an increase in pyrylium character on salt formation, the effect being largest in (3). Increased positive charge on the pyran rings in (2) and (3) is confirmed by shifts in their 1H and 13C n.m.r. signals relative to (1), though these reveal no obvious distinction between the two salts. There is a relatively short [2.34(4)A] linear N–H ⋯ Br hydrogen bond in the hydrobromide. Structural and spectroscopic data support an inversion mechanism for isomerisation about the CN bond in the imine (1), consistent with the observed low ΔG‡ value, while isomerisation in the two salts (with higher ΔG‡ values) is probably by a different mechanism.

Competing acyl transfer and intramolecular O ? N acyl group migration from an isolable O-acylisourea

The O-acylisourea (16a), which is a model for the proposed intermediate in carbodi-imide condensations, and is formed on reaction of the chloride (15) with benzoate ion, was shown using X-ray crystallography to have the Z-configuration. Crystals are monoclinic, space group P21/c with four molecules in a cell of dimensions a= 15.732(3), b= 16.246(5), c= 7.628(1)Å, β= 108.94(1)°. The structure was solved by direct methods and refined by full-matrix least-squares calculations to an R value of 0.044 for 1 281 observed reflexions. The benzoyl group is trans to the lone pair of the imine nitrogen and the CN bond length is 1.286(6)Å. The isourea undergoes uncatalysed (at 25° in 1 : 4 dioxan–water, µ= 1.0) O → N acyl transfer to give the N-acylamide (17) at pH > 7; the slow step is a Z→E isomerisation. At lower pH (3–6), Z→E isomerisation of the isourea (and thus O → N acyl transfer) is acid catalysed, but at pH < than the pKa of (16a), intramolecular acyl transfer is inhibited by protonation of the substrate. Competing intermolecular reaction of (16a) with the solvent [to give the urea (18)] occurs at all pH. In acid, reaction is between the protonated (16a) and water and in base between the free isourea (16a) and hydroxide ion. The O-acylisourea also undergoes rapid intermolecular acyl transfer to a variety of amines, although reaction between the free amine and protonated isourea was too slow to be observed. The implications of these results for the design of conditions to optimise inter-(rather than intra-) molecular reactions in carbodi-imide-mediated condensations are discussed.

Ab initio calculations for the ground state of nitrosomethane and the non-vertical n_.π* excitation energy

Abstract The molecular potential energy of nitrosomethane has been computed by SCF CI methods along three coordinates involving the CN and NO bond lengths and their bond angle. These potential energy curves may be used for force constant calculations. The vertical n_.π* excitation energy into the lowesttA″ state was calculated to be 17 030 cm−1. Corrections due to geometry changes upon electronic excitation imply an upper limit of 14 530 cm−1 for the non-vertical separation energy of the two potential energy minima. This is in excellent agreement with the experimental origin of the visible spectrum at 14 408 cm−1.

HCNO as a semirigid bender

The semirigid bender Hamiltonian [Bunker and Landsberg, J. Mol. Spectrosc., 67, 374–385 (1977)] is used to fit the rotation-vibration energy level separations in the fulminic acid (HCNO) molecule. The allowance made in the model for the variation of the CH and CN bond lengths with the HCN bending angle proves to be very important, and as well as achieving a good fit we are able to make a detailed investigation of the shape of the HCN bending potential function. From the results we conclude that the equilibrium structure of HCNO is linear but that excitation of the ν 1 or ν 2 stretching vibrations gives rise to an effective HCN bending potential function having its minimum at a nonlinear configuration. Even in the ground state the zeropoint vibrational contributions from ν 1 and ν 2 to the effective HCN bending potential give a small barrier (11.5 cm −1) to linearity, and we determine that the zero-point HCN bending vibrational amplitude is ±34°.

ChemInform Abstract: STEREOELECTRONIC PROPERTIES OF TETRAHEDRAL SPECIES DERIVED FROM CARBONYL GROUPS. AB INITIO STUDY OF AMINODIHYDROXYMETHANE, CH(OH)2NH2, A MODEL TETRAHEDRAL INTERMEDIATE IN AMIDE HYDROLYSIS

An ab initio theoretical study of all fifteen fully staggered conformations of aminodihydroxymethane, CH(OH)2NH2 has been performed. Optimization of the CO and CN bond lengths, population analyses and orbital localisation reveal the presence of marked conformation dependent stereoelectronic effects which influence bond lengths and overlap populations. These effects may be parametrized as a function of number and nature of antiperiplanar (app) oriented electronic lone pairs (1p) and polar bonds. In a YCX fragment an app orientation between a lone pair on Y and the CX bond increases the length and weakens the CX bond, shortens and strengthens the CY bond. Thus a CX (X O, N) bond of CH(OH)2NH2 is longest and weakest when: (i) it is app to two vicinal lp's; (ii) the X 1p's are not app to a vicinal polar bond; (iii) the conformation of the molecule has as many axially oriented lp's as possible. Results (i) and (ii) agree with a simple hyperconjugation picture involving interaction between an electronic 1p and an app oriented antibonding bond orbital σ* (CX). Bond properties, relative energies and effects on reactivity of the tetrahedral species are discussed, as well as their relations to experimental results on the cleavage of tetrahedral intermediates and to enzyme catalysis.

Theoretical dipole moment derivatives and force constants for HCN

Valence configuration interaction wavefunctions are used to calculate dipole moment derivatives and force constants for linear hydrogen cyanide. The magnitude of the dipole moment is found to decrease when the C–N bond is stretched from its equilibrium value. The implications for hydrogen bonding in crystalline HCN are discussed. Self-consistent field calculations are used to confirm that the SCF dipole derivative with respect to the CN bond length has the wrong sign and to explore the sensitivity of the dipole derivatives to the choice of atomic basis set.

The C—H⋯O hydrogen bond. An ab initio study of nitrosomethane and its hydrogen bonded dimer

In order to study the short C—H⋯O contact which has been found in several nitroso compounds, a series of ab initio calculations have been performed on nitrosomethane and it's cyclic “hydrogen bonded” dimer. A potential function for the C—H⋯O contact has been found and the effect of this contact upon the NO and CN bonds has been studied. The potential is shallow with a minimum of only −2.65 kcal mol −1 for each contact and the equilibrium C⋯O distance is 3.524, A. These results indicate that the C—H⋯O bond is better described as a van der Waal's type contact than a hydrogen bond. The equilibrium length of the NO bond ( R NO) changes in a regular manner with variations in the C⋯O ( R HYD) distance, i.e. when R HYD becomes shorter R NO becomes longer. However, the variations in the CN bond lengths, which in the nitrosomethane monomer molecule is a long and weak bond, are anomalous.

ChemInform Abstract: A COMPARATIVE AB INITIO STUDY OF SOME MOLECULAR SYSTEMS BASED ON HYDRAZINE AND FORMAMIDE

Abstract A series of ab initio calculations has been performed on formamide and its hydrogen bonded dimer, s-diformylhydrazine, s-dimethylhydrazine and hydrazine. All systems were assumed planar. The CO, CN and NN bond lengths were optimized in all cases, in order to study the changes of these bonds with variations in the environment. Gross atomic populations and force constants were also considered, and in some calculations the effect of basis enlargements was studied. Significant changes were found in the formamide unit upon dimerization via hydrogen bonds or a NN bond. And, also, a systematic variation was found for the NN bond by going from hydrazine via dimethylhydrazine to diformylhydrazine. The results of this study indicate that the effect of substitution within the framework used is well simulated by ab initio calculations.

Excited electronic states of HNC, hydrogen isocyanide

Walsh’s rules suggest that the excited states of HNC should have bond angles similar to the analogous states of HCN. To test this hypothesis, ab initio calculations have been carried out and compared to earlier calculations on HCN. The most surprising result is the prediction that several of the excited electronic states of HNC lie below the corresponding states of HCN. Also unanticipated are the unusually long CN bond lengths found for several of the lower excited states of HNC. The excited singlet states have bond angles qualitatively similar, although somewhat smaller, than those of HCN. For the triplet states, Walsh’s concept appears less successful, primarily owing to strong mixing between several different electronic configurations. Mulliken populations are used in the discussion of these results.

The effect of hydrogen bonding on structural parameters I: An ab initio study of the keto and enol tautomers of formamide and their hydrogen bonded dimers

In order to study the variation of CO and CN bond lengths as functions of the hydrogen bond length, a series of ab initio calculations have been performed on the keto and enol tautomers of formamide. The formation of hydrogen bonds leads to an increase in the conjugation of the NCO fragment. This increase is expressed as a lengthening of the double bonds and a corresponding shortening of the single bonds. These changes are found to vary with the length of the hydrogen bonds and analytical expressions for these variations of the bond lengths have been derived. The potential functions for dimerization, i.e. formation of, respectively, two N-H ·· O and two O-H ·· N hydrogen bonds have also been found. The results obtained indicate significant differences between the two types of hydrogen bonds.