Potential energy profiles of the geometric isomerization and the thermal decomposition of diphosphene HP=PH in the ground and excited electronic states

Summary The geometric isomerization and the dehydrogenation of HP=PH in the ground and some low-lying excited states are investigated by theoretical calculations. The reaction paths are traced by either the CASSCF or UHF-SCF calculations using the 6-31G(d,p) basis functions, and the accompanying energy changes are calculated by the MRD-CI method employing the [5s3p1d]/[2s1p] basis functions. The barrier heights for the trans-to-cis isomerization, by the planar inversion and the nonplanar twisting, in the ground state are calculated to be 265 and 144 kJ/mol (with the vibrational zero-point energy corrections), respectively. The latter barrier is noticeably lower than the H-P and the P-P bond dissociation energies oftrans-HP=PH (¹A_g), which are 304 and 271 kJ/mol, respectively. The ground-state HP₂ radical (²A'), which is to be formed by the dehydrogenation of HP=PH, should suffer further decomposition into P₂ (¹ _g ⁺ ) and H with an activation energy of 139 kJ/mol. The lowest excited state of HP₂ is found to be a hydrogen-bridged 3-electron system (²A₂) having an isosceles triangle structure. It has proved to be formed by the dehydrogenation of the lowest excited singlet state (¹B) of HP=PH via a transition state which lies 194 kJ/mol above the¹B state. The excited HP₂ (²A₂) is state-correlated with P₂ (³_u)+H.     

Synthese und Konfiguration von Methylestern der diastereomeren (±)-3-Amino-2,3-diarylpropansäuren,der (±)-3-Amino-2,3-diarylpropanole und ihrer Derivate

Zusammenfassung Durch Umsetzung von Hydramiden mit am aromatischen Ring m,p-dialkoxysubstituierten Arylessigsäureestern in Gegenwart von wasserfr. AlCl₃ entstehen Gemische aus Estern der (±)-erythro- und (±)-threo-3-Amino-2,3-diarylpropansäuren. Bei der Umsetzung von Piperonalhydramid mit dem Methylester der Homoveratrumsäure konnte lediglich daserythro-Diastereomere isoliert werden.Die diastereomeren Ester wurden zu den jeweiligen (±)-erythro- und (±)-threo-3-Amino-2,3-diaryl-1-propanolen reduziert, die als N,O-Dibenzoylderivate charakterisiert wurden. Ihre relativen Konfigurationen und die der zugehörigen Ester wurden auf Grund der IR-Spektren der diastereomeren Aminopropanole bestimmt. Die NMR-Spektren eines Diastereomerenpaares von Aminopropanolen bestätigen die ihnen chemisch zugeordneten Konfigurationen und ermöglichen den Nachweis auch ihrer bevorzugten Konformationen.

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Synthesis and configuration of methyl esters of the diastereomeric (±)-3-amino-2,3-diarylpropanoic acids, the (±)-3-amino-2,3-diaryl-1-propanols and their derivatives

Reaction of hydramides with m,p-dialkoxysubstituted arylacetates in the presence of anhydrous AlCl₃ generally leads to mixtures of the esters of the (±)-erythro- and (±)-threo-3-amino-2,3-diarylpropanoic acids. However, in the reaction of piperonal hydramide with methyl homoveratrate, only the correspondingerythro-diastereomer was obtained.Reduction of the diastereomeric esters gives the corresponding (±)-erythro- and (±)-threo-3-amino-2,3-diaryl-1-propanols, which can be characterized as N,O-dibenzoyl derivatives. Relative configurations of both alcohols and esters were determined by means of their IR spectra. Configurations were confirmed for one pair of diastereomeric amino alcohols by means of NMR spectra and preferred conformations deduced.

     

Crystal engineering: Supramolecular structures by cocrystallization ofMeso-1,2-diphenyl-1,2-ethanediol and bisimines,simple cases of molecular recognition

The crystal structures of molecular complexes betweenmeso- 1,2-diphenyl-1,2-ethanediol and two bisimines (N,N-(dibenzylidene)-ethylenediamine and glyoxylidene-bis(2,4-dimethyl-3-pentyl-amine) are reported at different temperatures. The structure-determining motif of the cocrystalline arrangement is one single O-H . N hydrogen bond resulting in infinite ladderlike polymers. The supramolecular structure is formed by recognition of fitting species: Thed- orl-isomers do not arrange in such structures.¹H NMR experiments show that no prearrangements take place by forming complexes in solution.     

Electronic structure and the unimolecular reactions of imine peroixde HNOO

Summary Electronic structure and possible unimolecular reaction paths of a linear four-atom molecule HNOO to be formed by the addition of NH(^3–) toward O₂(³ _g ^– ) are investigated by the SCF and MRD-CI calculations employing the 6–31G** basis functions. HNOO in its ground state (¹ A) is an ozone-like diradicaloid, whose N–O binding energy is only 27 kJ/mol. Geometries and excitation energies of various diradical (excited) states, both singlet and triplet, are examined. The isomerization paths of the ground-state HNOO(¹ A) are traced by a multi-configuration (MC) SCF procedure and the activation barrier heights evaluated by the CI treatment. It has proved that energetically the most favorable is the 1,3-hydrogen migration to give hydroperoxynitrene NOOH(¹ A) with the barrier height of 62 kJ/mol. The nitrene should be extremely unstable; it is liable to be decomposed to NO + OH with virtually no activation barrier.Presented at the 7th International Congress on Quantum Chemistry, Menton, July 1991     

Correlated rotation of aryl substituents in diarylmethyl-, diarylphosphine- and related fragments. An empirical force field study

Summary Ground state structures and conformational interconversion mechanisms of 25 diaryl compoundsAr ₂ Z (Z=CH₂, CHR, CH(OH), P-CH₃) were analyzed. For tetra(ortho-alkyl)substituted diaryls the cogwheeling mechanism was found as the threshold mechanism. A shift from the cogwheeling mechanism to interconversions via 2-ring flips is found in di(ortho-alkyl)substituted compounds. The ground state structures and interconversion mechanisms of diarylmethylphosphines are very similar to those of the related 1,1-diarylethanes. The interconversion barrier for correlated conrotation of the aryl rings in di(tert-butylphenyl)methanol (20) was measured by low temperature NMR and is in excellent agreement with the calculated value for the 2-ring flipT₂ (G (exp.)=48 kJ mol^–1; G (calc.)=54 kJ mol^–1).

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Korrelierte Rotation von Arylringen in Diarylmethyl-, Diarylphosphin- und verwandten Fragmenten. Eine Untersuchung mit Hilfe der empirischen Kraftfeldmethode

Zusammenfassung Die Grundzustandskonformationen und die konformativen Interkonversions-mechanismen von 25 DiarylverbindungenAr ₂ Z (Z=CH₂, CHR, CH(OH), P-CH₃) wurden analysiert. Für tetra(ortho-alkyl)substituierte Diaryle wurde der cogwheeling-Mechanismus als der Interkonversionsmechanismus niedrigster Energie ermittelt. In di(ortho-alkyl)substituierten Verbindungen werden nicht der cogwheeling-Mechanismus sondern 2-ring flips als Interkonversionsmechanismen gefunden. Die Grundzustände und Interkonversionsmechanismen für Diarylmethylphosphine sind sehr ähnlich jenen der verwandten 1,1-Diarylethane. Die Interkonversionsbarriere für die korrelierte Bewegung der Arylringe von Di(tert-butylphenyl)methanol (20) wurde mittels Tieftemperatur-NMR-Spektroskopie ermittelt und ist in sehr guter Übereinstimmung mit dem berechneten Wert für den 2-Ring flipT₂ (G (exp.)=48 kJ mol^–1; G (calc.)=54 kJ mol^–1).

     

Studies of artificial hydrolytic metalloenzymes: The catalyzed carboxyester hydrolysis by copper(II), zinc(II) and cobalt(II) complexes of the tripod ligand tris(2-benzimidazylmethyl)amine

The hydrolysis kinetics of p-nitrophenyl acetate (NA) catalyzed by Cu^II, Zn^II and Co^II complexes of tris(2-benzimidazylmethyl)amine (NBT) have been studied. The hydrolysis rate is first-order in both metal(II) complex and NA. The second-order rate constants, k_cat are 0.083, 0.241 and 0.285mol^–1Ls^–1 (298K, I = 0.10molL^–1 KNO₃, 0.02molL^–1 tris buffer, 40% MeCN aqueous solution) for Zn–NBT, Co–NBT and Cu–NBT complexes, respectively. The result indicates that the hydrolytic metalloenzyme activity of different metal complexes increases with the electrophilicity of the metal ions and that the complexes, in this paper, constitute that most efficient hydrolytic metalloenzyme models reported to date. An increase in MeCN content in the solution greatly reduces the hydrolytic activity of the nucleophiles.     

Lipase-mediated deracemization of secondary 1-phenyl-substituted propargylic alcohols of different topology

Acetylation of (±)-1-phenylnon-2-yn-1-ol, (±)-1-phenylhept-1-yn-3-ol, and (±)-1-phenylundec-4-yn-3-ol ((±)-5) in the presence of lipase from Candida cylindracea (CCL) proceeds slowly to give products with ee 20%. The acetates of these alcohols are hydrolyzed in the presence of porcine pancreatic lipase (PPL) equally unsatisfactorily. The (⁶-arene)tricarbonylchromium complex of alcohol (±)-5 is acetylated in the presence of CCL up to 22% conversion to give (R)-acetate whose oxidative decomplexation followed by saponification results in alcohol (R)-(–)-5 with ee 95%. The configuration of alcohols (–)-5 and (+)-5 was determined by NMR spectroscopy of their esters with (R)- and (S)-Mosher"s acids.     

Synthesis of bimodal molecular weight distribution polyethylene with α-diimine nickel(II) complexes containing unsym-substituted aryl groups in the presence of methylaluminoxane

A series of unsymmetrical complexes of 2,3-bis(2-phenylphenyl)-butanediimine nickel(II) dibromide (complex 1), 1,4-bis(2-isopropyl-6-methylphenyl)-acenaphthenediimine nickel(II) dibromide (complex 2) and meso- and rac-1,4-bis (2,4-di-tert-butyl-6-methylphenyl)-acenaphthenediimine nickel(II) dibromide (meso-3 and rac-3) were synthesized and activated by methylaluminoxane (MAO) for ethylene polymerization. By ¹³C NMR characterization, meso- and rac-stereo-isomers were detected in the condensation products resulting from the reaction of unsym-substituted anilines with diketones. It was notable that meso- and rac-isomers in ligand 1 or ligand 2 could not be separated owing to their interconversion, however, meso- and rac-isomers in ligand 3 could be isolated and identified by X-ray diffraction and NMR analysis. At low polymerization temperatures, complex 1/MAO afforded polyethylene with bimodal molecular weight distribution, while complex 2/MAO prepared polyethylene with single-modal distribution. Moreover, by raising polymerization temperature or extending time of catalyst aging, bimodal molecular weight distribution polyethylene was also produced by complex 2/MAO. The hypothesis of bimodal molecular weight distribution polyethylene synthesized by unsymmetrical α-diimine nickel(II) complexes was supported that the molecular weight of polyethylene produced by rac-3/MAO was significantly higher than that produced by meso-3/MAO under identical polymerization conditions. A unique methodology to prepare polyethylene with bimodal molecular weight distribution was demonstrated.     

Tautomerism and stereochemistry of hypericin: Force field,NMR, and X-ray crystallographic investigations

Summary Stereochemistry and tautomerism of hypericin, pseudohypericin, and several of their partial structure models were investigated using an MM2 derived force field method. Besides the propeller type conformer, which was also found by the X-ray crystallographic study, the complicated energy hypersurface was shown to contain a novel double-butterfly conformer of similar stability. The upper limit interconversion barrier between these conformers and their enantiomers was found to be in the order of 115 kJ/mol.¹H-NMR experiments suggested a lower limit interconversion barrier of at least 80 kJ/mol. From the ten tautomers possible in principle, the 7,14-species was derived to be the most stable one by at least 48 kJ/mol.

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Tautomerie und Stereochemie von Hypericin: Untersuchungen mit Hilfe der Kraftfeld-Methodik, NMR-Spektroskopie und Röntgenstrukturanalyse

Zusammenfassung Die Stereochemie von Hypericin, Pseudohypericin und einigen seiner Partialstrukturmodelle wurde mit Hilfe einer von MM2 abgeleiteten Kraftfeldmethodik untersucht. In der komplizierten Energiehyperfläche wurde neben dem auch durch Röntgenstrukturanalyse gefundenen Propeller-Konformeren ein neues Doppelschmetterling-Konformer ähnlicher Stabilität aufgefunden. Die obere Grenze für die Interkonversionsbarrieren zwischen diesen Konformeren und ihren Enantiomeren sind in der Größenordnung von 115 kJ/mol. Aus¹H-NMR-Experimenten konnte eine untere Grenze von wenigstens 80 kJ/mol abgeleitet werden. Es wurde gefunden, daß von den zehn prinzipiell möglichen Tautomeren die 7,14-Spezies die um wenigstens 48 kJ/mol stabilste ist.

     

Excitation of Chemiluminescence as a Reversal of Radiationless Transition: Semiempirical Simulation of Decomposition of Peroxides

Excitation of chemiluminescence is treated as a reversal of radiationless transition. The thermolysis of six 1,2-dioxetanes was examined by the semiempirical PM3 method. The major contribution to the reaction coordinate is made first by the stretching of the O–O bond and then by the cleavage of the C–C and O–O bonds. The energy profile is characterized by a shallow well (4–12 kJ/mol) in the region of the biradicaloid transition state. Here, the potential energy surfaces of the ground state and the triplet state are close to one other, and according to the Hund rule, the state T₁ lies lower than S₀. The well prolongs the lifetime . Therefore, the yield of chemiexcitation of the triplet product k _isc may be sufficiently high even at a low value of the rate constant k _isc of the spin-forbidden transition from S₀ to T₁. Methodological problems and the specifics of a quantum-chemical computer experiment (reliability of the Dewar semiempirical methods, reproducibility, and systematic and random errors) are considered.     

Multidimensional tunneling dynamics of proton transfer in malonaldehyde molecule and its isotopomers

The twenty-one-dimensional Hamiltonian of malonaldehyde molecule and a number of its isotopomers (H/D, ¹³C/¹²C) was reconstructed in the low-energy region (<3000 cm^–1). Parameters of the Hamiltonian were obtained from quantum-chemical calculations of the energies, equilibrium geometries, and eigenvectors and eigenfrequencies of normal vibrations at the stationary points corresponding to the ground state and transition state. Despite substantial variation of the barrier height calculated using different quantum-chemical methods (from 2.8 to 10.3 kcal mol^–1), the corresponding potential energy surfaces can be matched with high accuracy by scaling only one parameter (the semiclassical parameter , which defines the scales of potential, energy, and action). Scaling invariance allows optimization of the Hamiltonian in such a way that the calculated ground-state tunneling splitting coincides with the experimental value. The corresponding potential barrier height is estimated at 4.34±0.4 kcal mol^–1. The quantum dynamics problem was solved using the perturbative instanton approach without reducing the number of degrees of freedom. The role of all transverse vibrations in proton tunneling is characterized. Vibration-tunneling spectrum is calculated for the ground state and low-lying excited states and mode-specific isotope effects are predicted.     

Equilibrium between α- and β-isomers of dihydroartemisinine and its multiple-peaks in high-performance liquid chromatography

Summary The interconversion between the - and -isomers and the multiple-peaks of dihydroartemisinine (dihydroqinghaosu) were investigated in reversed-phase, highperformance liquid chromatography. A convenient method using the Gaussian distribution function instead of the Schmidt graphical method for the theoretical calculation of the diffuse probability curves versus the fraction of the dead time on column as form was introduced. Based on the theoretical probability curves and the experimental multiple-peak chomatograms, the effects of temperature and flow velocity on the accuracy and precision of analysis of dihydroartemisinine are indicated and the activation energy of the interconversion between - and -isomers of dihydroartemisinine is estimated.     

Über 3,4-Bis-(p-hydroxyphenyl)-hexandiole-(2,5) und ihre 2,5-Dimethylhomologen

Zusammenfassung Von den in der vorigen Mitteilung beschriebenen 3,4-Bis-(p-methoxyphenylhexandionen-(2,5) [meso undracem.] wurde diemeso-Verbindung durch Reduktion in zwei (der drei möglichen) entsprechende sekundäre Diole übergeführt und das höherschmelzende Diol durch Entmethylierung in ein 3,4-Bis-(p-hydroxyphenyl)-hexandiol-(2,5) (Dihydroxyhexoestrol) umgewandelt. Versuche, durch Reduktion eine Überführung in ein Hexoestrol zu erreichen und die sterische Anordnung festzulegen, schlugen sämtlich fehl.Eingehender wurde die Gewinnung homologer tertiärer Diole durch Umsetzung der beiden Hexandionderivate (meso undracem.) mit CH₃MgX bearbeitet, wobei unter Ätherspaltung die beiden Bis-homologen-dihydroxy-hexoestrole resultierten. Ammesoiden Hexandionderivat wurde auch die Grignardreaktion allein (ohne Entmethylierung) durchgeführt.

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Of the two (meso andracemic) 3,4-di(p-methoxyphenyl)-hexanediones-(2,5) described in the previous communication, themeso compound has been converted by reduction into two (of the three possible) corresponding secondary diols. Demethylation of the higher melting isomer yielded 3,4-di(p-hydroxyphenyl)-hexanediol-(2,5) (dihydroxyhexestrol). Attempts to obtain the corresponding hexestrol by reduction and hence to determine its configuration remained unsuccessful.The preparation of homologous tertiary diols by reaction of the two hexanedione derivatives (meso andracemic) with CH₃MgX has been investigated more closely; on ether cleavage the two di-homologous dihydroxyhexestrols were formed. The Grignard reaction was also carried out on themeso hexanedione without subsequent demethylation.

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6. Mitt.:H. Bretschneider undR. Lutz, Mh. Chem.95, 1702 (1964).     

Molecular Structure and Internal Rotation Potential of Dimethylphenylphosphine, According to Gas-Phase Electron Diffraction Data and Quantum-Chemical Calculations

Geometric parameters of dimethylphenylphosphine molecule were determined by gas-phase electron diffraction using a dynamic model in which the rotation of the PMe₂ group is treated as large-amplitude motion. Refinement of the structural parameters and parameters of the potential function was performed taking into account the geometry relaxation on the basis of HF/6-311++G** calculations. The internal rotation potential has a single minimum at 0° ( is the angle between the bisector of the MePMe angle and the phenyl ring plane) and may be described by the function of the form V() = 0.5V ₂(1 - cos2), where V ₂ = 0.38±0.36 kcal mol^{- 1}. The data obtained are compared with those for related molecules. Steric effects affect the geometry of the phenylphosphine molecule more significantly than does p- interaction.     

Reinterpretation of Dynamic Vibrational Spectroscopy to Determine the Molecular Structure and Dynamics of Ferrocene

Molecular distortion of dynamic molecules gives a clear signature in the vibrational spectra, which can be modeled to give estimates of the energy barrier and the sensitivity of the frequencies of the vibrational modes to the reaction coordinate. The reaction coordinate method (RCM) utilizes ab initio‐calculated spectra of the molecule in its ground and transition states together with their relative energies to predict the temperature dependence of the vibrational spectra. DFT‐calculated spectra of the eclipsed (D_5h) and staggered (D_5d) forms of ferrocene (Fc), and its deuterated analogue, within RCM explain the IR spectra of Fc in gas (350 K), solution (300 K), solid solution (7–300 K), and solid (7–300 K) states. In each case the D_5h rotamer is lowest in energy but with the barrier to interconversion between rotamers higher for solution‐phase samples (ca. 6 kJ mol^?1) than for the gas‐phase species (1–3 kJ mol^?1). The generality of the approach is demonstrated with application to tricarbonyl(η⁴‐norbornadiene)iron(0), Fe(NBD)(CO)₃. The temperature‐dependent coalescence of the ν(CO) bands of Fe(NBD)(CO)₃ is well explained by the RCM without recourse to NMR‐like rapid exchange. The RCM establishes a clear link between the calculated ground and transition states of dynamic molecules and the temperature‐dependence of their vibrational spectra.     

The effect of mechanical grinding on the formation and crystallinity changes of the inclusion complex of acetaminophen andβ-cyclodextrin

The effect of mechanical grinding on the physicochemical properties of acetaminophen in the presence of three additives,- or-cyclodextrin and microcrystalline cellulose, was studied by using TLC, powder X-ray diffraction analysis, infrared spectroscopy and differential scanning calorimetry. The results indicate that the crystallinity of physical mixtures of acetaminophen and the described additives decreased with increased grinding time and formed an amorphous state when mixtures containing- or-cyclodextrin were ground with acetaminophen. We also found that the acetaminophen molecules could be included step-by-step into the cavity of-cyclodextrin molecules and formed an amorphous inclusion complex.-Cyclodextrin and microcrystalline cellulose did not form an inclusion complex with acetaminophen, but acted only to decrease the crystallinity of the ground mixtures. The mechanical grinding efficiency for acetaminophen was improved in the order of-cyclodextrin -cyclodextrin > microcrystalline cellulose.This paper is part XI of Drug Interaction in Pharmaceutical Formulations.     

Conformational composition of the 2,4,6-trimethyl-1,3,2-dioxaborinane stereoisomers

A study of conformational transformations of 2,4,6-trimethyl-1,3,2-dioxaborinane cis- and trans-isomers in non-empirical quantum-chemical HF/6-31G (d) and PBE/3z approximations showed that the transformation routes include the sofa conformers with different orientations of the substituents at the ring atoms C⁴ and C⁶ through the transition states corresponding to diequatorial, axial-equatorial, and dixial conformations of the 2,5-twist form. The calculation and ¹H NMR data show that the conformational equilibrium of cis-isomer is almost entirely shifted to diequatorial sofa, and the trans-form is characterized by the interconversion between the two axial-equatorial sofa conformers.     

Stereochemie von Metallocenen, 43. Mitt.: Torsionsisomere Biphenyl-tricarbonylchrom-Komplexe: Darstellung,Konformationsanalyse und Kinetik der Interkonversion

Complexation of methyl 3-nitro-2-(o-tolyl)-benzoate (2, ano-trisubstituted biphenyl) with hexacarbonyl chromium gave the isomeric Cr(CO)₃ complexes3 and4 both existing as two torsional isomersa andb, as seen by nmr. For the main product3 the preferred conformations of the two torsional isomersa andb were deduced by the lanthanide induced shift (LIS) technique. The activation energy for the interconversion process could be determined from kinetic measurements and especially from¹H-nmr coalescence temperature of the methylsignals. The barrier of 91±1.5 kJ/mol (21.7±0.3 kcal/mol) in3 reflects the interaction of theo-H-atom of the benzene ring passing the Cr(CO)₃ moiety of the benchrotrene ring during the interconversion.A partial optical resolution of3 was achieved by asymmetric reduction with a chiral lithium-aluminium hydride yielding (+)-3 with [] _D ²⁰ +30° (benzene) and the levorotatory alcohol (–)-7.

42. Mitt.:K. Schlögl undR. Schölm, Mh. Chem.109, 1227 (1978).     

Structure of Charge Transfer Reaction Complexes Formed in Anionic Polymerization of Isoprene

A new mechanism is suggested for the anionic polymerization of isoprene. The key moment of this mechanism is thermal electron excitation of the complex of a living polymer with a monomer to the low lying S₁ (T₁) state involving a charge (electron) and (Li⁺) cation transfer from the terminal unit to the monomer molecule. It is stated that the probability of chemical bonding depends on the spin density on the radical centers of reactant molecules and on the geometry of the reaction complex. The semiempirical AM1 and ab initio 6-31G* quantum-chemical calculations revealed strong interaction for the ground electronic state of the complex (5-10 kcal/mole) and low energies of the excited triplet levels (<10 kcal/mole).     

Barriers to internal rotation in phosphoric amide molecules

Internal rotation in phosphoramide and 0,0-dimethyl phosphoramidate molecules in their different stable conformations was studied by different quantum-chemical methods. Rotation of the amide group about the P-N bond is inhibited with barrier values of 27 kJ/mole.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 2, pp. 221–22, March–April, 1985.