Structure and rearrangements of 3-Iso(thio,seleno)cyanato-1,2,3-triarylcyclopropenes

A series of new fluctional 3-iso(thio,seleno)cyanato-1,2,3-triarylcyclopropenes was synthesized. The structure of compounds was proved by ¹H and ¹³C NMR, IR, and mass spectra, and that of 3-(1,2,3-triphenylcyclopropenyl) isothiocycnate was confirmed by X-ray crystallography. In compounds under consideration by means of ¹H and ¹³C NMR was discovered and investigated a fast reversible migration of isocyanato, isothiocyanato, and isoselenocyanato groups along the perimeter of the theree-membered ring proceeding according to the dissociation-recombination mechanism.     

New approach to preparation of N-acylphosphoramido(thio)(seleno)ates

N-[2-(X)-1,3,2-Oxathiaphospholane] derivatives (X = S, Se, O) of carboxamides were prepared and their DBU-assisted reaction with alcohols led to the corresponding O-alkyl-N-acylphosphoramido(thio)(seleno)ates. Their structures were confirmed by MS analysis and ¹H and ³¹P NMR spectroscopy. Independently N-acylphosphoramidoselenoates were converted to N-acylphosphoramidates by treatment with tert-butylperoxytrimethylsilane. The oxathiaphospholane approach was also applied to the synthesis of derivatives having N-prolylphosphoramido(thio)(seleno)ate linkages on the 5′-OH group of AMP.     

Synthesis of novel thio(seleno) phosphate-phosphonates and their biological activities

In order to search for novel herbicides with high activity and selectivity, a series of thio(seleno) phosphate-phosphonate derivatives were synthesized in good yields by a one-pot procedure using tris(diethylamino)phosphine activated by iodine as the phosphorylating reagent. The structures of all compounds prepared were confirmed by spectroscopic methods and microanalyses. Preliminary bioassays indicated that some of the products possess potent and selective herbicidal activities and antiphytoviral activities against the tobacco mosaic virus (TMV). In addition, some of them exhibit significant fungicidal activities against R. Solani and P. Piricola. © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:369–375, 1998     

Electronic structure and conformational properties of the amide linkage Part 5. Internal rotation and inversion in 1-formylaziridine

Internal rotation and nitrogen inversion in 1-formylaziridine (1) have been investigated by quantum mechanical (ab initio and MNDO) calculations, especially with respect to the variation of the geometry of the aziridine ring. While conformational stability is mainly determined by the n(N)/π(CO) interaction, the bond lengths within the ring are affected by the amount of interaction between the π(CO) orbital and the Walsh orbital ω_A. To separate the two types of interaction, calculations were also performed on formylcyclopropane (9). The torsional potential of 1 has a minimum close to the perpendicular conformation 1b. The two bisected conformations, 1a and 1c, are transition states for internal rotation. For nitrogen inversion, a barrier of 1.44 kcal mol⁻¹ (ab initio) was calculated. Calculations on 1-cyanoaziridine (7) gave inversion barriers of 5.81 (ab initio) and 12.31 kcal mol⁻¹ (MNDO). Probably due to methodical reasons the ab initio values seem to be too low, as calculations with different basis sets for aziridine indicate.     

Intramolecular and intermolecular contributions to the barriers for rotation of methyl groups in crystalline solids: electronic structure calculations and solid-state NMR relaxation measurements

The rotation barriers for 10 different methyl groups in five methyl-substituted phenanthrenes and three methyl-substituted naphthalenes were determined by ab initio electronic structure calculations, both for the isolated molecules and for the central molecules in clusters containing 8-13 molecules. These clusters were constructed computationally using the carbon positions obtained from the crystal structures of the eight compounds and the hydrogen positions obtained from electronic structure calculations. The calculated methyl rotation barriers in the clusters (E(clust)) range from 0.6 to 3.4 kcal/mol. Solid-state (1)H NMR spin-lattice relaxation rate measurements on the polycrystalline solids gave experimental activation energies (E(NMR)) for methyl rotation in the range from 0.4 to 3.2 kcal/mol. The energy differences E(clust) - E(NMR) for each of the ten methyl groups range from -0.2 kcal/mol to +0.7 kcal/mol, with a mean value of +0.2 kcal/mol and a standard deviation of 0.3 kcal/mol. The differences between each of the computed barriers in the clusters (E(clust)) and the corresponding computed barriers in the isolated molecules (E(isol)) provide an estimate of the intermolecular contributions to the rotation barriers in the clusters. The values of E(clust) - E(isol) range from 0.0 to 1.0 kcal/mol.     

"Amide resonance" correlates with a breadth of C-N rotation barriers

Complete basis set calculations (CBS-QB3) were used to compute the CN rotation barriers for acetamide and eight related compounds, including acetamide enolate and O-protonated acetamide. Natural resonance theory analysis was employed to quantify the "amide resonance" contribution to ground-state electronic structures. A range of rotation barriers, spanning nearly 50 kcal/mol, correlates well to the ground-state resonance weights without the need to account for transition-state effects. Use of appropriate model compounds is crucial to gain an understanding of the structural and electronic changes taking place during rotation of the CN bond in acetamide. The disparate changes in bond length (DeltarCO < DeltarCN) are found to be consonant with the resonance model. Similarly, charge differences are consistent with donation from the nitrogen lone pair electrons into the carbonyl pi* orbital. Despite recent attacks on the resonance model, these findings demonstrate it to be a sophisticated and highly predictive tool in the chemist's arsenal.     

Exocyclic push–pull conjugated compounds. Part 3. An experimental NMR and theoretical MO ab initio study of the structure, the electronic properties and barriers to rotation about the exocyclic partial double bond in 2-exo-methylene- and 2-cyanoimino-quinazolines and -benzodiazepines

The structure of a number of 2-exo-methylene substituted quinazolines and benzodiazepines, respectively, 1, 3a,b, 4 (X=–CN,–COOEt) and their 2-cyanoimino substituted analogues 2, 3c,d (X=–CN,–SO₂C₆H₄–Me(p) was completely assigned by the whole arsenal of 1D and 2D NMR spectroscopic methods. The E/Z isomerism at the exo-cyclic double bond was determined by both NMR spectroscopy and confirmed by ab initio quantum chemical calculations; the Z isomer is the preferred one, its amount proved dependent on steric hindrance. Due to the push–pull effect in this part of the molecules the restricted rotation about the partial C²,C¹¹ and C²,N¹¹ double bonds, could also be studied and the barrier to rotation measured by dynamic NMR spectroscopy. The free energies of activation of this dynamic process proved very similar along the compounds studied but being dependent on the polarity of the solvent. Quantum chemical calculations at the ab initio level were employed to prove the stereochemistry at the exo-cyclic partial double bonds of 1–4, to calculate the barriers to rotation but also to discuss in detail both the ground and the transition state of the latter dynamic process in order to better understand electronic, inter- and intramolecular effects on the barrier to rotation which could be determined experimentally. In the cyanoimino substituted compounds 2, 3c,d, the MO ab initio calculations evidence the isomer interconversion to be better described by the internal rotation process than by the lateral shift mechanism.     

Theoretical Study of Electronic Structures and Spectra of Chalcogenido Complexes of Molybdenum, trans-Mo(Q)(2)(PH(3))(4) (Q = O, S, Se, Te)

Electronic structures of the title complexes have been studied using quantum chemical computations by different methods. It is shown that the results of Xalpha calculations agree well with expectations from classical ligand-field theory, but both are far from being in agreement with the results given by ab initio calculations. The HOMO in the ab initio Hartree-Fock molecular orbital diagrams of all these complexes is a chalcogen p(pi) lone pair orbital rather than the metal nonbonding d(xy)() orbital previously proposed. Electronic transition energies were calculated by CASSCF and CI methods. The results suggest that in the cases when Q = S, Se, and Te the lowest energy transitions should be those from the p(pi) lone pair orbitals to the metal-chalcogen pi orbitals. The calculated and observed electronic spectra of the oxo complex are in good agreement and very different from the spectra of the other complexes, and the lowest absorptions were accordingly assigned to transitions of different origins.     

Conformational consequences of the dynamic processes in the stereolabile atropisomers of acyl-substituted m-terphenyl derivatives

By means of low-temperature NMR spectroscopy, conformers (stereolabile atropisomers) due to the restricted rotation about the Ar-Ar and Ar-C(O)R bonds were detected in a number of acylphenyl derivatives, substituted in positions 2 and 6 by the 3-isopropylphenyl moiety (compounds 1-3, R=H, Me, and t-Bu, respectively). The conformational assignment was accomplished on the basis of the symmetry of the low-temperature 13C NMR spectra with the added support of ab initio calculations. The interconversion barriers were also determined by complete line shape simulation of the NMR spectra, and the experimental values were satisfactorily reproduced by ab initio calculations. In the case of the asymmetric derivative 4, two enantiomers, generated by the restricted t-BuC(O)-Ar rotation, were found sufficiently stable to allow their separation by means of the enantioselective HPLC technique at ambient temperature and to obtain the corresponding CD spectra.     

Solvent effects on the thioamide rotational barrier: an experimental and theoretical study.

The solvent effect on the C-N rotational barriers of N,N-dimethylthioformamide (DMTF) and N,N-dimethylthioacetamide (DMTA) has been investigated using ab initio theory and NMR spectroscopy. Selective inversion recovery NMR experiments were used to measure rotational barriers in a series of solvents. These data are compared to ab initio results at the G2(MP2) theoretical level. The latter are corrected for large amplitude vibrational motions to give differences in free energy. The calculated gas phase barriers are in very good agreement with the experimental values. Solvation effects were calculated using reaction field theory. This approach has been found to give barriers that are in good agreement with experiment for many aprotic, nonaromatic solvents that do not engage in specific interactions with the solute molecules. The calculated solution-phase barriers for the thioamides using the above solvents are also in good agreement with the observed barriers. The solvent effect on the thioamide rotational barrier is larger than that for the amides because the thioamides have a larger ground-state dipole moment, and there is a larger change in dipole moment with increasing solvent polarity. The transition-state dipole moments for the amides and thioamides are relatively similar. The origin of the C-N rotational barrier and its relation to the concept of amide "resonance" is examined.     

Reaction mechanism of the acidic hydrolysis of highly twisted amides: Rate acceleration caused by the twist of the amide bond

We present an ab initio study of the acid hydrolysis of a highly twisted amide and a planar amide analogue. The aim of these studies is to investigate the effect that the twist of the amide bond has on the reaction barriers and mechanism of acid hydrolysis. Concerted and stepwise mechanisms were investigated using density functional theory and polarizable continuum model calculations. Remarkable differences were observed between the mechanism of twisted and planar amide, due mainly to the preference for N-protonation of the former and O-protonation of the latter. In addition, we were also able to determine that the hydrolytic mechanism of the twisted amide will be pH dependent. Thus, there is a preference for a stepwise mechanism with formation of an intermediate in the acid hydrolysis, whereas the neutral hydrolysis undergoes a concerted-type mechanism. There is a nice agreement between the characterized intermediate and available X-ray data and a good agreement with the kinetically estimated rate acceleration of hydrolysis with respect to analogous undistorted amide compounds. This work, along with previous ab initio calculations, describes a complex and rich chemistry for the hydrolysis of highly twisted amides as a function of pH. The theoretical data provided will allow for a better understanding of the available kinetic data of the rate acceleration of amides upon twisting and the relation of the observed rate acceleration with intrinsic differential reactivity upon loss of amide bond resonance.     

烷基自由基β位裂解反应类反应势垒与速率常数的精确计算

提出反应类等键方法并用于高温燃烧机理中一类重要反应——烷基自由基β位裂解反应的反应势垒和速率常数的精确校正计算. 通过10种不同从头算水平对类反应中5个代表反应的反应势垒的计算发现, 用反应类等键反应方法和直接从头算方法获得的5 个代表反应的反应势垒最大绝对偏差的平均值分别为5.32 和16.16 kJ·mol^-1, 表明反应类等键反应方法计算的反应势垒对不同水平从头算方法的依赖性小, 可在较低从头算水平计算得到精确的反应势垒, 解决大分子体系反应势垒的精确计算问题. 此外应用反应类等键反应方法在BHandHLYP/cc-pVDZ 从头算水平计算了3 个代表反应的速率常数, 并与文献报道的实验值进行了比较, 其在500-2000 K温度区间内计算速率常数与实验速率常数中较大值与较小值的比值k_max/k_min的平均值为1.67, 最大值也仅有2.49. 表明应用反应类等键反应方法在较低从头算水平即可对同类反应的速率常数进行精确计算.最后在BHandHLYP/cc-pVDZ从头算水平用反应类等键反应方法计算了13个烷基自由基β位裂解反应的速率常数.     

Anion-pi interactions in cyanuric acids: a combined crystallographic and computational study

Several structures of pi complexes of isocyanuric acid and of several thio derivatives with anions have been computed by using high level ab initio calculations. The nature of the complexes has been studied by means of the method of molecular interaction potential with polarization (MIPp) and Bader's theory of atoms-in-molecules. These molecules form favorable complexes with anions and can be used as binding units for building receptors for the molecular recognition of anions. In several cases, the anion-pi interaction has been demonstrated experimentally by means of X-ray crystallography.     

Conformational characterization of lanthanide(III)-DOTA complexes by ab initio investigation in vacuo and in aqueous solution

The conformational behavior of four [Ln(DOTA)(H(2)O)](-) systems (Ln = La, Gd, Ho, and Lu) has been characterized by means of ab initio calculations performed in vacuo and in aqueous solution, the latter by using the polarizable continuum model (PCM). Calculated molecular geometries and conformational energies of the [Ln(DOTA)(H(2)O)](-) systems, and interconversion mechanisms, barriers, and (13)C NMR spectra of the [Lu(DOTA)](-) complex are compared with experimental values. For each system, geometry optimizations, performed in vacuo and in solution at the HF/3-21G level and using a 46+4f(n) core electron effective core potential (ECP) for lanthanides, provide two minima corresponding to a square antiprismatic (A) and an inverted antiprismatic (IA) coordination geometry. All the systems are nonacoordinated, with the exception of the IA isomer of the Lu complex that, from in solution calculations, is octacoordinated, in agreement with experimental data. On comparing the in vacuo relative free energies calculated at different theory levels it can be seen that the nonacoordinated species dominates at the beginning of the lanthanide series while the octacoordinated one does so at the end. Furthermore, on passing along the series the IA isomer becomes less and less favored with respect to A and for the Lu complex a stabilization of the IAisomer is observed in solution (but not in vacuo), in agreement with the experimental data. Investigation of the A<-->IA isomerization process in the [Lu(DOTA)](-) system provides two different interconversion mechanisms: a single-step process, involving the simultaneous rotation of the acetate arms, and a multistep path, involving the inversion of the cyclen cycle configuration. While in vacuo the energy barrier for the acetate arm rotation is higher than that involved in the ring inversion (23.1 and 13.1 kcal mol(-)(1) at the B3LYP/6-311G level, respectively), in solution the two mechanisms present comparable barriers (14.7 and 13.5 kcal mol(-)(1)), in fairly good agreement with the experimental values. The NMR shielding constants for the two isomers of the [Lu(DOTA)](-) complex have been calculated by means of the ab initio GIAO and CSGT methods, and using a 46-core-electron ECP for Lu. The calculated (13)C NMR chemical shifts are in close agreement with the experimental values (rms 3.3 ppm, at the HF/6-311G level) and confirm the structural assignment of the two isomers based on experimental NMR spectra in solution. The results demonstrate that our computational approach is able to predict several physicochemical properties of lanthanide complexes, allowing a better characterization of this class of compounds for their application as contrast agents in medical magnetic resonance imaging (MRI).     

Quantification of the (anti)aromaticity of fulvalenes subjected to pi-electron cross-delocalization

Fulvalenes 3-12 were theoretically studied at the ab initio level of theory. For the global minima structures, the occupation of the bonding (pi)C=C orbital of the interring C=C double bond obtained by NBO analysis quantitatively proves pi-electron cross-delocalization resulting in, at least partially, 2- or 6pi-electron aromaticity and 8pi-electron antiaromaticity for appropriate moieties. The cross-conjugation was quantified by the corresponding occupation numbers and lengths of the interring C=C double bonds, while the aromaticity or antiaromaticity due to cross-delocalization of the pi-electrons was visualized and quantified by through-space NMR shielding surfaces.     

Predicting experimental complexation-induced changes in (1)H NMR chemical shift for complexes between zinc-porphyrins and amines using the ab initio/GIAO-HF methodology

Ab initio calculations were carried out on zinc-porphyrins complexed to several amines: N-(3,5-dimethyl-pyridin-4-yl)-formamide, 1,4-diazabiciclo[2.2.2]octane (DABCO), and 1-azabiciclo[2.2.2]octane (quinuclidine). The proton chemical shifts of these complexes were calculated ab initio at the GIAO-HF/6-311G//HF/3-21G level of theory, and the obtained values agree satisfactorily with experimental results. The complexation-induced changes in (1)H NMR chemical shifts correlate well with differences in association constants of several host-guest complexes.     

Rotational spectrum of a chiral alpha-hydroxyester: conformation stability and internal rotation barrier heights of methyl lactate

High resolution spectrum of methyl lactate, a chiral alpha-hydroxyester, has been investigated using a molecular jet Fourier transform microwave spectrometer. High level ab initio calculations were employed to study the conformational isomerism of methyl lactate. The observed rotational spectrum confirms that the most stable conformer has an intramolecular hydrogen bond of OH...O==C type, as predicted by the ab initio calculations. The internal rotation barrier heights of the ester methyl group and the alpha-carbon methyl group were calculated to be 5.4 and 14.5 kJ mol(-1) at the MP2/aug-cc-pVDZ level of theory for the most stable conformer. The internal rotation splittings due to the ester methyl group were observed and analyzed and the ester methyl group tunneling barrier height was determined experimentally to be 4.762 (3) kJ mol(-1).     

Ab initio calculation of torsion and inversion barriers of the amino group in aminopyrimidines

Calculations of the barriers to internal rotation and inversion of the amino group in substituted pyrimidines have been performed. Torsion and inversion barriers were determined by several ab initio methods: HF, HF/MP2, MP4, CISD, QCISD, QCISD(T), CCSD, and CCSD(T). DFT method also employed. Dependencies of the calculated barrier heights on the basis set and the electron correlation level and on the substitution position of the nitrogen atom in the ring were studied. We have determined that for certain molecules relatively low level calculations may eventually provide adequate results, but in general, higher level calculations are necessary.     

Barriers to rotation in methyl formate by dynamic NMR spectroscopy and barriers to 1,3 oxygen-to-oxygen migration in methyl formate and trifluoromethyl formate by ab initio calculations

Free-energy barriers of 9.85 and 11.91 +/- 0.15 kcal/mol at -70.8 degrees C were found by dynamic NMR spectroscopy for the E-to-Z and Z-to-E conversions, respectively, of methyl formate (1) enriched in 13C to 99% for the carbonyl carbon [methyl formate 13C (2)]. These barriers are higher than the literature values reported for -53 degrees C. The free-energy barrier to 1,3 oxygen-to-oxygen migration of the methyl group in methyl formate was determined by ab initio calculations at several levels. The value of 58.7 kcal/mol obtained at the MP2/6-311+G (df,pd) level was compared to a literature barrier for this process (MINDO/3) and to barriers for related compounds. A free-energy barrier of 63.0 kcal/mol for the oxygen - to - oxygen migration of the CF3 group in trifluoromethyl formate (3) was calculated at the MP2/6-31+G level.