RHF Levels Research Articles

Conformational preferences of the substrates of lactate dehydrogenase

The substrates of lactate dehydrogenase (pyruvate and lactate) as well as an inhibitor (oxamate) are characterized using ab initio methods employing the basis sets 6-31+G(d) and 6-31+G(d,p) at both RHF and MP2 levels. The first aim of these studies is to explore the conformational preferences of these molecules and to compare the minimum energy conformation with that found in crystallographic structures of oxamate bound in the enzyme active site. In the lowest energy conformer of pyruvate, the carboxyl group is rotated about 80° from the plane defined by the carbonyl group and methyl carbon atom, a preference dominated by repulsion between the carboxyl and carbonyl oxygens. A relatively small barrier of ∼2kcal/mol for rotation of the carboxyl group is calculated at the MP2/6-31+G(d,p) level. In contrast, both oxamate and lactate prefer a nearly planar orientation of the carboxyl group, which is more like the enzyme-bound conformation of oxamate. In these molecules, the repulsion between the oxygen atoms is counterbalanced by intramolecular hydrogen bonds, which are maximized in a near-planar conformation. The barrier to carboxyl rotation in oxamate is ∼4kcal/mol at the MP2/6-31+G(d,p) level, while that for lactate is much higher, about 10kcal/mol at the RHF/6-31+G(d) level. Thus, even though the minimum-energy lactate conformation is more similar to the enzyme-bound oxamate conformation than that for pyruvate, a steeper conformational energy surface indicates that at least as much energy would be required for the smaller torsional changes which occur on binding.A second aim of these studies is to evaluate whether the semiempirical methods AM1 and PM3 commonly used in QM/MM calculations of enzyme reactions adequately reproduce higher-level ab initio conformational results. The AM1 method is shown to be accurate for pyruvate and lower-energy regions of lactate, but less accurate for oxamate. PM3 tends to overestimate barriers to conformational changes for these molecules.

Anab initio study of excited states of guanine in the gas phase and aqueous media: Electronic transitions and mechanism of spectral oscillations

Ground state geometries of the four tautomeric forms keto-N9H, keto-N7H, enol-N9H, and enol-N7H of guanine were optimized in the gas phase at the RHF level using a mixed basis set consisting of the 4-31G basis set for all the atoms except the nitrogen atom of the amino group for which the 6-311+G* basis set was used. These calculations were also extended to hydrogen-bonded complexes of three water molecules with each of the keto-N9H (G9-3W) and keto-N7H (G7-3W) forms of guanine. Relative stabilities of the four above-mentioned tautomers of guanine as well as those of G9-3W and G7-3W complexes in the ground state in the gas phase were studied employing the MP2 correlation correction. In aqueous solution, relative stabilities of these systems were studied using the MP2 correlation correction and polarized continuum model (PCM) or the isodensity surface polarized continuum model (IPCM) of the self-consistent reaction field (SCRF) theory. Geometry optimization in the gas phase at the RHF level using the 6-31+G* basis set for all atoms and the solvation calculations in water at the MP2 level using the same basis set were also carried out for the nonplanar keto-N9H and keto-N7H forms of guanine. Thus, it is shown that among the different tautomers of guanine, the keto-N7H form is most stable in the gas phase, while the keto-N9H form is most stable in aqueous solution. It appears that both the keto-N9H and keto-N7H forms of guanine would be present in the ground state, particularly near the aqueous solution–air interface. Vertical excitation and excited state geometry optimization calculations were performed using configuration interaction involving single electron excitation (CIS). It is found that the absorption spectrum of guanine would arise mainly due to its keto-N9H form but the keto-N7H form of the same would also make some contribution to it. The enol-N9H and enol-N7H forms of the molecule are not expected to occur in appreciable abundance in the gas phase or aqueous media. The normal fluorescence spectrum of guanine in aqueous solution with a peak near 332 nm seems to originate from the lowest singlet excited state of the keto-N7H form of the molecule while the fluorescence of oxygen-rich aqueous solutions of guanine with a peak near 450 nm appears to originate from the lowest singlet excited state of the keto-N9H form of the molecule. The origin of the slow damped spectral oscillation observed in the absorption spectrum of guanine has been explained. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 826–846, 2000

Metal-assisted coupling of oximes and nitriles: a synthetic, structural and theoretical study

Chlorination of [Ph3PCH2Ph][PtCl3(EtCN)], obtained from the reaction of [PtCl2(EtCN)2] with [Ph3PCH2Ph]Cl, formed the platinum(IV) complex [Ph3PCH2Ph][PtCl5(EtCN)] which, at ambient temperature and both in solution and in the solid phase, hydrolyses to the ammonia compound [Ph3PCH2Ph][PtCl5(NH3)] and undergoes nucleophilic addition by ketoximes or amidoxime HONCR1R2 [R1R2 = Me2, C4H8, C5H10, C9H16, C9H18 or Ph(NH2)] to give the corresponding iminoacylated product [Ph3PCH2Ph][PtCl5{HNC(Et)ONCR1R2}]. All compounds were characterized by elemental analyses, FAB mass spectrometry, IR and 1H, 13C-{1H}, 31P-{1H} and 195Pt NMR spectroscopies. A crystal structure determination of [Ph3PCH2Ph][PtCl5{NHC(Et)ONC(C9H16)}] disclosed amidine one-end rather than the N,N-bidentate co-ordination mode of the N-donor ligand. The iminoacylation by oximes was investigated by ab initio methods (at RHF level using quasi-relativistic pseudopotentials for platinum) for [PtCl5(NCMe)]− which were also applied to the related neutral platinum(IV) [PtCl4(NCMe)2] and platinum(II) [PtCl2(NCMe)2] complexes. The calculations included geometry optimization of the starting and final complexes, location of possible transition states for the reaction discussed and intrinsic reaction coordinate calculations for one reaction. The results obtained provided an interpretation, on the basis of kinetic (activation energies) and thermodynamic (reaction energies) effects, for the order of reactivity observed [neutral PtIV > anionic PtIV > neutral PtII] and indicated that a mechanism based on nucleophilic addition of the protic nucleophile (undeprotonated oxime), to form a transition state with a four-membered NCOH ring, is energetically favoured relative to the alternative one involving prior deprotonation of the oxime, unless base-catalysed conditions are operating.

The thermolytic behaviour of hexa-1,5-diene-units incorporated in a stellane and twistbrendane skeleton. A quest for a stepwise Cope rearrangement

The thermolytic behaviour of the tricyclo[3.3.0.03,7]octane (stellane) derivatives 11–13 as well as the tricyclo[4.3.0.03,8]nonane (twistbrendane) derivatives 14 and 15 have been investigated in d8-toluene. All five compounds undergo Cope rearrangements to the tricyclic systems 16–20. By means of 1H NMR spectroscopy the kinetics of these reactions have been studied at different temperatures. The lowest activation energies were encountered for the reactions of 12 and 13 (27.0 and 26.3 kcal mol−1, respectively). Model calculations on the RHF and CASSCF level and beyond (CASPT2N/6-31G*//RHF/6-31G*; CASPT2N/6-31G*//CASSCF/6-31G*) were performed for the rearrangements of 11→16, 13→18, and 27→28. These calculations reveal that the activation energies for the two-step pathway and the concerted pathway are very similar. In the case of 11 and 27 the two step mechanism via a bisallylic diradical intermediate is predicted to be slightly preferred.

Anisole as an Ambidentate Ligand: Ab Initio Molecular Orbital Study of Alkali Metal Cations Binding to Anisole

The authors present optimized geometries and binding energies for alkali metal cation complexes with anisole (methoxybenzene). Results are obtained for Li{sup +} through Cs{sup +} at the RHF/6-311G* and MP2/6-311+G* levels of theory, with K{sup +}, Rb{sup +}, and Cs{sup +} represented by relativistic ECPs and associated valence basis sets. RHF/6-311G{sup {minus}} frequencies are used to verify that the optimized geometries are minima and to calculate binding enthalpies. The effects of basis set superposition error (BSSE) are estimated at both the RHF and MP2 levels. The alkali metals bind to anisole in two ways, either predominantly through interactions with the aromatic ring or with the ether oxygen. For binding to the ring, BSSE-corrected MP2/6-311+G* binding enthalpies (in kcal/mol) of {minus}38.1 (Li{sup +}), {minus}23.6 (Na{sup +}), {minus}18.3 (K{sup +}), {minus}15.4 (Rb{sup +}), and {minus}13.6 (Cs{sup +}) were obtained. The average distances (in {angstrom}) between the ring carbons and the cations are 2.33 (Li{sup +}), 2.79 (Na{sup +}), 3.20 (K{sup +}), 3.44 (Rb{sup +}), and 3.70 (Cs{sup +}). For binding to the ether oxygen, the BSSE-corrected MP2/6-311+G* binding enthalpies (in kcal/mol) are {minus}37.6 (Li{sup +}), {minus}25.2 (Na{sup +}), {minus}19.4 (K{sup +}), {minus}16.4 (Rb{sup +}), and {minus}14.3 (Cs{sup +}). The distances (in {angstrom})more » between the ether oxygen and the cations are 1.82 (Li{sup +}), 2.24 (Na{sup +}), 2.62 (K{sup +}), 2.87 (Rb{sup +}), and 3.10 (Cs{sup +}). Although the differences in binding energy between the two sites are small, the cations generally prefer to bind to the oxygen.« less

Theoretical molecular structures for trifluoromethyl hypohalites, CF 3OX (X=F, Cl, Br): ab initio and DFT calculations

Ab initio and density functional theory (DFT) calculations using the Gaussian 94 program have been performed to investigate the equilibrium molecular structures, rotational barriers, vibrational spectra, and 19F NMR chemical shifts of trifluoromethyl hypohalites, CF 3OX (X=F, Cl, Br), with the basis sets larger than 6-31G*. For each molecule the staggered form is found to be more stable than the eclipsed form. The eclipsed conformer corresponds to the transition state structure of which energy barrier is around 4∼5 kcal/mol. In general, the calculations at the B3LYP level can reproduce the experimental molecular properties of CF 3OX better than those at the RHF level.

The potential function for the internal rotation and the derived potential energy distribution of the normal modes for 3-chloropropionyl chloride based on ab initio calculations

The conformational behavior and structure of 3-chloropropionyl chloride were investigated by ab initio calculations. The 6-311++G** basis set was employed to include polarization and diffuse functions in the calculations at RHF and MP2 levels. The compound was predicted to exist as a complex mixture of stable conformers. The trans– cis conformation with minimum chlorine–chlorine interaction was predicted to be the lowest energy form for the molecule. From the potential scan, the barrier for internal rotation between the trans– cis and trans– gauche conformers was calculated. The vibrational frequencies were computed at the HF level. Normal coordinate calculations were carried out and potential energy distributions among symmetry coordinates were calculated for the stable conformers of the molecule. Raman and infrared spectra of both the individual conformers and the mixture of them were computed at 300 K.

Water Complexes and Hydrolysis of Silicon Tetrafluoride in the Gas Phase: An ab Initio Study

An ab initio study of the structure, binding energies, electronic distribution, vibrational frequencies, and thermodynamic parameters of possible gas-phase complexes of silicon tetrafluoride with water (1:1, 2:1, and 1:2) in different conformations has been performed at the RHF, B3LYP, MP2, and MP4(sdq) levels with a variety of basis sets up to cc-pVTZ. On this basis, the thermodynamic stability, assignment of observed infrared bands of SiF4/H2O adducts in matrixes and in the gas phase, and activation energies of the elementary reactions leading to polyhydroxy derivatives and siloxane are discussed. The most thermodynamically stable structure is a 1:1 complex with the most accurate estimate of the binding energy of 2.7 kcal/mol. This complex is characterized by a small amount of electron transfer from the water molecule to silicon tetrafluoride and by small participation of the silicon d-orbitals in the coordination bond. A single stable conformation has been found for complex 2SiF4·H2O whereas the complex SiF4·2H2O can exist in two conformations (distorted octahedral symmetry), distinguished by positions of the water molecules. The energetically preferred conformation of SiF4·2H2O is that with the water molecules in cis positions of an octahedron. The activation energy for the hydrolysis of first Si−F bond is 21 kcal/mol. The most thermodynamically preferred product at the first stages of hydrolysis is the hexafluorodisiloxane SiF3−O−SiF3 (activation energy 30 kcal/mol) rather than polyhydroxy derivatives.

Carbon−Carbon Triple Bonds as Nucleophiles: Adducts of Ethyne and Propyne with Boron Trifluoride

The mid-infrared (4000−400 cm-1) spectra of ethyne/BF3 and propyne/BF3 mixtures, dissolved in liquefied argon are discussed. In all spectra, the formation of a 1:1 van der Waals complex was observed, while at higher concentrations of BF3 absorption bands of a 1:2 species between propyne and BF3 were also observed. Using spectra recorded at several temperatures between 96 and 125 K, the complexation enthalpy ΔH° for ethyne·BF3 was determined to be −9.3(4) kJ mol-1. For propyne·BF3 and propyne·(BF3)2 the ΔH° values are −13.2(4) and −19.8(12) kJ mol-1, respectively. For the 1:1 complexes the structures and harmonic vibrational frequencies were calculated at the MP2/6-31+G(d) level, while for the 1:2 adduct similar calculations were carried out at the corresponding RHF level. Using a SCRF/SCIPCM scheme to correct for the solvent influences, and statistical thermodynamics to account for the zero-point vibrational and thermal contributions, approximate values for the gas-phase complexation energy were obtained ...

Shapes of Molecules by Millimeter-Wave Spectroscopy: 2-Phenylethanol

We have detected one conformer of 2-phenylethanol and determined its shape using Stark-modulated free jet microwave spectroscopy. By comparing observed and ab initio predicted spectroscopic rotational constants and hydroxyl-deuteration isotopic substitution coordinates, the species has been identified with a gauche conformer, designated as 1, in which the hydroxyl hydrogen atom is involved in an intramolecular hydrogen bond with the π-electron cloud of the aromatic ring. Stark effect measurements yielded electric dipole moment values of μa = 1.251(3) D and (μb2 + μc2)1/2 = 1.102(10) D, and hence μtotal = 1.667(10) D. Previously published molecular mechanics and RHF level ab initio calculations predict that other conformers are of comparable stability to 1. However, when electron correlation is included at the MP2/6-31G(d,p) level, the predicted energy of 1 is substantially (ca 8 kJ mol-1) lower than that of all the other conformers. This is in full accord with our identification of only one conformer in our spectroscopic searches at a pre-expansion temperature of 363 K.

Beryllium-9 NMR Study of Solid Bis(2,4-pentanedionato-O,O‘)beryllium and Theoretical Studies of 9Be Electric Field Gradient and Chemical Shielding Tensors. First Evidence for Anisotropic Beryllium Shielding

Despite the favorable NMR properties of 9Be (I = 3/2), NMR spectroscopy of this nucleus in the solid state remains comparatively unexplored, perhaps owing to the extreme toxicity of beryllium and its compounds. We present here an integrated experimental and theoretical study of the Be chemical shielding (CS) and electric field gradient (EFG) tensors in bis(2,4-pentanedionato-O,O‘)beryllium [Be(acac)2]. Interpretation of the 9Be NMR data was facilitated by crystal X-ray diffraction results, which indicate two crystallographically unique sites (Onuma, S.; Shibata, S. Acta Crystallogr. 1985, C41, 1181). Beryllium-9 NMR spectra acquired at 4.7 and 9.4 T for magic-angle spinning (MAS) and stationary samples have been fitted in order to extract the nuclear quadrupole coupling constant (CQ), asymmetry parameter (η), and isotropic chemical shift (δiso). The best-fit nuclear quadrupole parameters for the two sites were determined to be CQ(1) = −294 ± 4 kHz, η(1) = 0.11 ± 0.04; CQ(2) = −300 ± 4 kHz, η(2) = 0.15 ± 0.02. Our analyses of the stationary samples also reveal a definite anisotropy in the beryllium CS tensor and allow us to place upper and lower limits on the spans of 7 and 3 ppm. This is the first evidence for anisotropic shielding in beryllium. Ab initio calculations of the beryllium CS tensors in Be(acac)2 at the RHF level indicate spans ranging from 7 to 9 ppm; this represents a substantial fraction of the total known chemical shift range for Be (<50 ppm). The calculated CQs are also in good agreement with the experimental results. To put the Be(acac)2 results in context, calculations of the beryllium CS tensors for a series of compounds encompassing the known range of 9Be chemical shifts are also presented. The calculations are in outstanding accord with experimental data from the literature. On the basis of calculations for linear molecules, it is shown that the assumption that the 9Be chemical shift is governed essentially by the diamagnetic term is erroneous. For some of these molecules, the calculated Be CS tensor spans are greater than the total known chemical shift range.

Inelastic Neutron Scattering Spectrum and Quantum Mechanical Calculations on the Internal Vibrations of Pyrimidine

A vibrational analysis of the inelastic neutron scattering spectrum (INS) of pyrimidine has been carried out combined with quantum mechanical calculations at the RHF, MP2, and B3LYP levels using the 6-31G* basis set in all the cases. The INS spectrum was first calculated from the atomic displacement matrix in order to determine which level of theory gives the best agreement between observed and calculated INS intensities. In a second approach, force constants from a calculation at the MP2/6-31G* level were transformed to a symmetrized set (C2v) and all the diagonal and some off-diagonal force constants were fitted. Good agreement between observed and calculated wavenumbers and spectral INS intensities was achieved which demonstrates the validity of our force field.

Theoretical investigation of the gas phase tautomerization of sarcosine

The proton transfer reaction involving the neutral intramolecular hydrogen bonded conformer of sarcosine (N-methylglycine), Iu, to form the zwitterionic tautomer, Iz, was analyzed by theoretical chemistry. Ab initio calculations show that the zwitterionic conformer Iz, is a minimum at all RHF levels of theory examined. The formation of Iz involves a proton transfer from Iu to form Iz, via a transition state, which is zwitterionic like. Form the RHF/6-311++G** calculations, the activation barrier for the tautomerization is 91.7kJ/mol, and the energy difference between Iz and the transition state, Its, is 1.9kJ/mol. Zwitterion Iz reverts to the unionized tautomer at levels of theory which include electron correlation.

Ab initio mechanistic investigation of samarium(III)-catalyzed olefin hydroboration reaction

The reaction path for Sm(III)-catalyzed alkene hydroboration reaction by catecholborane has been investigated using ab initio MO methods. The stationary structures on the model reaction path considering ethylene as alkene, Cp 2SmH as an active catalyst, and HB(OH) 2 as model borane were obtained at the RHF and MP2 levels, and the MP4SDQ energy calculations were carried out at the MP2 structures. In the reaction, initially ethylene coordinates to the active catalyst to form a π-complex. Then, ethylene insertion into the Sm–H bond takes place leading to stable Cp 2SmC 2H 5 after passing through the barrier of 4.2 kcal/mol. In the following step the model borane adds to Cp 2SmC 2H 5 to form a borane complex which thereafter passes through the small barrier of 1.1 kcal/mol giving rise to a product complex. In the final step, the dissociation of the hydroborated product, C 2H 5B(OH) 2 takes place with a large endothermicity of 40.4 kcal/mol. Because of the small activation energies, the rate determining step may be the last step. This reaction mechanism is different from that for Rh(I)-catalyzed hydroboration. The effects of the Sm atom on this energy profile is discussed. In addition, the bonding features of all the stationary structures on the reaction path are obtained from topological analysis of the corresponding electron density distributions.

Representation of parity violating potentials in molecular main chiral axes

We outline an approach to electroweak quantum chemistry, which leads to an increase of E pv by more than order of magnitude (at the CIS–RHF level), compared to established Hartree–Fock approaches. A unique axis system of molecular main chiral axes can be derived by transformation of the tensor || E pv ij || to diagonal form. In this form the dependence of E pv on nuclear charge is E pv∝ Z 4.2±0.3 for H 2O 2 and H 2S 2 without ambiguity arising from the molecular axis system. We discuss the relation to the evolution of homochirality, spectroscopic measurements of Δ E pv and the fundamental constants in the standard model.

Geometry and Electronic Structure of Titanacycloalkenes

A number of titanacycloalkenes, H2TiC2H2, HTiC3H3, H2TiC4H4, and HTiC5H5, have been studied using ab initio techniques. Vibrational frequencies were computed at the RHF, MP2, BLYP, and B3LYP levels. Both a LANL2DZ pseudopotential and an all-electron TZV basis set was used for each method. The electronic structure of the systems of interest is characterized in terms of natural resonance theory (NRT) and the atoms in molecules (AIM) analysis. All rings are planar except for titanabenzene, HTiC5H5. H2TiC2H2 has C2v symmetry and has single Ti−C bonds. HTiC3H3, unlike the isovalent cyclobutadiene and silacyclobutadiene, has C2v symmetry and shows significant electron delocalization. In H2TiC4H4 the TiC4 ring is planar with equivalent single Ti−C bonds and has C2v symmetry. HTiC5H5 has a boat-shaped geometry where the angle between the CTiC plane and the plane formed by the neighboring four carbon atoms is nearly 90°. AIM analysis isolates a weak critical point between Ti and the para-C atom, and NRT analysis c...

Ab Initio Molecular Orbital Study of Cation−π Binding between the Alkali-Metal Cations and Benzene

We present optimized geometries and binding energies for alkali-metal cation complexes with benzene. Results are obtained for Li+ through Cs+ at the RHF/6-311G* and MP2/6-311+G* levels of theory with K+, Rb+, and Cs+ represented by relativistic ECPs and associated valence basis sets. RHF/6-311G* frequencies are used to verify the optimized geometries are minima and used to calculate binding enthalpies. The effects of basis-set superposition error (BSSE) are estimated at both the RHF and MP2 levels. We obtain BSSE-corrected MP2/6-311+G* binding enthalpies (in kcal/mol) of 35.0 (Li+), 21.0 (Na+), 16.0 (K+), 13.3 (Rb+), and 11.6 (Cs+). The distances (Å) between the center of the benzene ring and the cation are 1.865 (Li+), 2.426 (Na+), 2.894 (K+), 3.165 (Rb+), and 3.414 (Cs+). Additional single-point CCSD(T)/6-311+G*//MP2/6-311+G* and CCSD(T)/6-311+G(2d,2p)//MP2/6-311+G* calculations indicate that the MP2/6-311+G* results are well converged with regard the extent of electron correlation, whereas small changes in binding energy are still observed when larger basis sets are used. Additional calculations using local and nonlocal density functional theory are included for comparison.

Solvation of the Menshutkin Reaction: A Rigorous Test of the Effective Fragment Method

The recently developed effective fragment potential (EFP) method is used to study the effect of two, four, six, and eight solvating water molecules on the Menshutkin reaction between ammonia and methyl bromide. The EFP method reproduces all ab initio geometries and energetics (including zero-point energy, thermal, and entropy effects) for the two-water case very accurately. Energetics from all ab initio single-point energies at the EFP geometries for the four, six, and eight water cases are in excellent agreement with corresponding EFP energetics. In the gas phase, the above Menshutkin reaction is kinetically highly unfavorable with a free energy of activation (at 298.15 K) of 40.6 kcal/mol at the RHF level with a double-ξ basis set augmented with polarization and diffuse functions. An ion-pair product is found, in agreement with previous work, in which the bromide anion is hydrogen-bonded to an ammonium hydrogen, giving a free energy of reaction of 2.8 kcal/mol. The addition of solvating water molecules ...

Regio- and stereoselectivity in the Diels–Alder reaction of vinylallenes with acrolein: an ab initio study

To explore the regio- and stereoselectivity in Diels–Alder reactions of vinylallenes with acrolein, a parent vinylallene/acrolein system and a methyl-substituted vinylallene/acrolein system were studied. Ab initio calculations were used to identify eight transition state structures for each of the two Diels–Alder reactions at various computational levels (RHF/6-31G(d), RHF/6-311G(2d), B3LYP/6-311G(2d), and MP2/6-311G(2d)). The relative energies of the endo and exo transition states along with the regioselectivity have been determined from these calculations. In the parent vinylallene/acrolein system, the endo s-cis transition structure is the preferred stereoselectivity at all levels of theory, however, there is no regioselectivity. In the methyl substituted vinylallene/acrolein system, the endo s-trans transition state tends to compete with the endo s-cis transition state at the RHF levels of theory and is 1 kcal/mol more stable at the B3LYP/6-311G(2d) level of theory. Also, in the methyl-substituted system, there is now a definite preference for one regioisomer over the other. Both Diels–Alder reaction systems are asynchronous with the methyl-substituted system being more pronounced.

Ab initio SCF-MO study of the Staudinger phosphorylation reaction between a phosphane and an azide to form a phosphazene †

The lowest energy pathway for a Staudinger reaction between a phosphane and an azide is predicted at the RHF and DFT ab initio SCF-MO level to proceed via an s-cis intermediate 7, followed by cyclisation and elimination of N2 to form a phosphazene. When suitable stabilising substituents are present, 7 can instead isomerise to the isolable s-trans intermediate 9. Natural bond orbital perturbation theory analysis has been employed to identify the factors influencing the relative stability of the s-cis phosphazide and the s-trans isomers.