A size extensive energy functional derived from a double configuration interaction approach: the role of N representability conditions

Starting from a configuration interaction (CI) approach including only doubly excited configurations, the corresponding energy functional is modified by introduction of a topological factor in the normalization condition of the CI coefficients in such a way that it gets inherently size extensive. Constraints imposed by N representability conditions on the modified functional are discussed and lead to a specific choice of the topological factor. The basic variable in the modified energy functional is the second-order reduced density matrix parametrized in terms of CI-like coefficients. Test calculations for a variety of small molecules show that the numerical results obtained with the new functional are in very good agreement with those obtained from coupled cluster singles doubles calculations.     

Role of doubly excited states in calculations of optical activity

It is shown that doubly excited states play an important role in calculations of the optical activity of molecules with well-conjugated electron systems, such as the DNA bases. In some significant cases it is necessary to include a large number of excited states in the configuration interaction (CI ) to obtain a reliable, converging result. A new version of the CNDO/OPTIC method, which includes doubly excited states in the CI , is proposed. As an application, the electric transition moments in different pyrimidines are considered. The calculated results agree with experimental data and results obtained from ab initio calculations and INDO calculations using doubly excited states in the CI .     

A new method for the detection of racemization during peptide synthesis: stereoselective hydrolysis of diastereomeric peptides by leucine aminopeptidase

A suitably protected dipeptide of configuration L -D , e.g. Z-L -Ala-D -Ala is coupled with an all L alanine peptide, e.g. L -Ala-L -Ala-ONb

1 Abbreviations according to the IUPAC-IUB rules, ‘Symbols for Amino-Acid Derivatives and Peptides, Recommendations (1971)’'. see e.g. J. biol. Chemistry 247, 977 (1972). In particular the following abbreviations have been used: Z = benzyloxycarbonyl-, -ONb = p-nitrobenzyloxy-, -ONSu = succinimido-oxy-. Additional abbreviations are LAP = leucine aminopeptidase, DCCI = N,N′-dicyclohexylcarbodiimide, DMF = dimethylformamide.

. The blocking groups are removed and the free peptide hydrolyzed by leucine amino peptidase (E.C. 3.4.1.1). This enzyme shows absolute L -specificity for the penultimate peptide bond from the amino end and therefore cleaves only the all L peptide formed through racemization. The amount of free alanine determined by amino acid analysis gives a multiple of the degree of racemization. The sensitivity of the test allows 0.1% of (L -Ala)₄ to be detected in the synthesis of L -Ala-D -Ala-L -Ala-L Ala. Coupling of Z-L -Ala-D -Ala and Z-L -Ala-D -Phe with di- and trialanine peptides has been studied using DCCI and DCCI + 1-hydroxybenzotriazole as coupling reagents. The degree of racemization was around 80% for the coupling by DCCI in DMF but was reduced to 0.2–0.4% in the presence of 2 equivalents of 1-hydroxybenzotriazole. Coupling using the succinimide esters Z-L -Ala-D -Ala-ONSu and Z-L -Ala-D -Phe-ONSu resulted in 0.8 to 10% racemization, depending on the solvent and base used.     

Direct MRCI method for the calculation of relativistic many-electron wavefunctions. I. General formalism

The formalism of a quasi- or full-relativistic multireference CI method has been developed and implemented. The scheme is appropriate for the calculation of molecular systems in which the relativistic effects are of the same order of magnitude as the correlation contributions. In this contribution some important symmetry aspects of a relativistic many-electron wave function are discussed and the consequences for the CI matrix structure are shown. An efficient CI strategy in the form of a direct CI is presented, which avoids the construction of the whole CI matrix. Based on a determinantal expansion of molecular spinor products, the individual one- and two-electron molecular integrals are processed, and the molecular symmetry is easily accounted for by a proper linear combination of Slater determinants in the CI starting vector. For an efficient CI organization some powerful techniques of the graphical unitary group approach have been transferred to the relativistic case.     

X-ray photoelectron spectra of aryldiazo derivatives of transition metals

The X-ray photoelectron spectra of singly bent, doubly bent, and protonated doubly bent aryldiazonium complexes have been obtained and show that the RN₂ ligand is highly reduced. The correlation observed between binding energies and structure of the MN₂R moiety permit the assignment of the N(1s) photopeaks.     

Ab initio CI calculation of the molecular quadrupole moment of benzene

The molecular quadrupole moment of benzene has been calculated from ab initio CI wavefunctions of different levels of accuracy The calculated value from the wavefunction near the Hartree-Fock limit is reduced ≈10% in magnitude by the CI treatment.     

The configuration interaction method,and the triplet-singlet splitting in CH2

A configuration-interaction (CI) method in which the interaction matrix is never constructed has been investigated, following the original suggestion of Roos. Two methods have been used (1) for singlet states, which can be represented by a one determinant configuration of doubly occupied orbitals, CI with all singly and doubly excited configurations, (2) for states for which the restricted self-consistent field approximation is a single determinant, CI with all singly and doubly excited determinants. In case (2), the wavefunction may not be exactly an eigenfunction of S ². The methods were investigated using a double-zeta plus polarisation basis for CH₂. Both methods must give the same result for the lowest singlet ground state. Keeping the bond length fixed at 2.10 and 2.04 bohr respectively the bond angle for the singlet and triplet were found to be 100.8 ° and 132.0 °, with energies ?39.0312 a.u. and ?39.0563 a.u. respectively. These are the lowest variational energies obtained for these systems; the singlet-triplet splitting is thus predicted to be 15.4 kcal/mol.     

cis peak as probe to investigate the molecular structure. Application to the rotational isomerism of 2,5-diphenylethenyl(hetero)arenes

A combined theoretical and experimental study on rotational isomerism, excited electronic states structure and cis peak effect is presented for flexible conjugated molecules containing two styryl groups coupled by an unsaturated pentatomic ring (furan, thiophene, pyrrole, cyclopentadiene). The several conformers of the ground electronic state have been investigated by means of Hartree-Fock ab initio methods and density functional theory. The electronic spectra have been calculated with the CS INDO CI approach in the singly-excited scheme and in that involving multiple excitations. By means of the theoretical findings and of the analysis of the cis peak of UV-vis absorption spectra, it is shown that the stable rotamer of these molecules is the trans-trans conformer. The trans-cis conformer may have also a relative presence at room temperature, mainly in 2,5-(PhE)2P. The cis-cis form is the most destabilized. The importance of the doubly excited electronic configurations in the theoretical treatments of the excited electronic states, in particular of the one responsible for the cis peak, the 2(1)A1 state, is shown. Some useful considerations on the S1-->Sn transient spectra are reported.     

Ab initio study of the singlet-triplet splitting in reduced polyoxometalates

A valence bond analysis of the wave function of doubly reduced polyoxometales is presented, using the M₆O₁₉ Lindqvist structure as test case. By a unitary transformation of the delocalised valence orbitals to localised metal centred orbitals, the multiconfigurational wave function is mapped onto a valence bond function with three different types of configurations: the two electrons are on the same site, on neighbouring sites, or on next-nearest neighbour sites. The inspection of the relative weights of these configurations for triplet and singlet state shows that the triplet-coupled electrons are confined to a smaller volume, and hence have a higher energy than the singlet-coupled electrons. This is in line with the experimental observation that the doubly reduced polyoxometalates show non-mangetic behaviour.     

Über den einsatz makroporöser harze in der festphasen-peptidsynthese

On the model systems Gly-(P), Val-(P) and (Val-Acp)_n-(P) the DCCI coupling rates with Boc-Gly and Boc-Val-Acp on Merrifield-resins and 8% crosslinked macroporous resins were determined quantitatively and the influence of the two supports on the rate of peptide chain lengthening investigated. Furthermore the course of each synthesis on the two polymers of Eledoisin partial sequence Lys-Phe-Phe-Gly-Leu-Met-NH₂ was compared by analytical control. The results show, that macroporous resins with a low degree of crosslinking are suitable supports for the solid-phase-synthesis of oligopeptides.     

Coupling of electron and proton transport in photosynthetic membranes: molecular mechanism

Using the method of Modified Neglect of Diatomic Overlap (MNDO), the electronic structure of plastoquinol (PQH(2)) and plastoquinone (PQ) in neutral, singly (PQ(-)) and doubly (PQ(2-)) reduced states is studied. The conformational analysis performed on these molecules shows that in the lowest energy conformation, the angle between the first link of the tail backbone and the ring plane of neutral and singly reduced PQ and plastoquinol is nearly the same and differs by 15 degrees from that of doubly reduced PQ. Nevertheless, for all states of plastoquinone and for plastoquinol, the total energy changes by less than 0.2 eV when the studied angle is varied from 0 degrees to 180 degrees. As in Rhodobacter sphaeroides, the oxygen of the PQ ring is reported to form a hydrogen bond with a nitrogen in the ring of Histidine (His) L 190. The energy of the PQ-His complex was calculated for different redox states of PQ and for several values of the distance between the molecules (N-O distance from 0.2 to 0.5 nm). For every considered complexes, the total energy dependence on the proton position on the line connecting the N and O atoms was determined, to see if the hydrogen bond is formed. It is shown that for only singly reduced PQ this dependence has a symmetric two-well form, i.e. the hydrogen bond is formed. For neutral and doubly reduced PQ, the curve is also two-well but asymmetric, so that the proton is bound to His or to PQ, correspondingly.On the basis of these results, we propose the following scheme of electron-proton coupling. Negatively charged oxygens of PQ form H-bonds with proton donor groups of the surrounding protein and fix PQ in its pocket. While the negative charges of oxygens increase after quinone reduction, protons shift to PQ oxygens and form strong hydrogen bonds with them. Upon second PQ reduction, protons are torn away from surrounding amino acids and form covalent bonds with the quinol. Resulting PQH(2) detaches from its binding place and is replaced by a neutral PQ. The lacking protons on amino acids in the Q(B) pocket are replaced by a step-by-step transfer from the stroma bulk through the proton channels.     

Reducing CAS-SDCI space. Using selected spaces in configuration interaction calculations in an efficient way

A new method is presented, which allows an important reduction of the size of some Configuration Interaction (CI) matrices. Starting from a Complete Active Space (CAS), the numerous configurations that have a small weight in the CAS wave function are eliminated. When excited configurations (e.g., singly and doubly excited) are added to the reference space, the resulting MR-SDCI space is reduced in the same proportion as compared with the full CAS-SDCI. A set of active orbitals is chosen, but some selection of the most relevant excitations is performed because not all the possible excitations act as SDCI generators. Thanks to a new addressing technique, the computational time is drastically reduced, because the new addressing of the selected active space is as efficient as the addressing of the CAS. The presentation of the method is followed by two test calculations on the N(2) and HCCH molecules. For the N(2) the FCI results are taken as a benchmark reference. The outer valence ionization potentials of HCCH are compared to the experimental values. Both examples allow to test the accuracy of the MR-SDCI compared to that of the corresponding CAS-SDCI, despite the noticeable reduction of the CI space. The algorithm is suitable for the dressing techniques that allow for the correction of the size-extensivity error. The corrected results are also shown and discussed.     

The mass spectra of intramolecularly hydrogen bonded 2(1H)-quinoxalinone and 2H-1,4-benzoxazin-2-one

The mass spectra of those 3-phenacyl derivatives of 2(1H)-quinoxalinone and 2H-1,4-benzoxazin-2-one with an intramolecular hydrogen bonding showd that: (1) the primary elimination of the shole sidchain is reduced as compared with an unchelated derivative; (2) the importan t fragmentation routes in the quinoxalinone series are ring cleavage with loss of CO followed by recyclization and expulsion of a hydrogen atom, and this can also occur in the reverse order; (3) this reverse process is virtually absent in the benzoxazinone series. Finally, the doubly charged molecular ions appear to undergo an analogous decomposition giving fairly intense [M-CO]²⁺ or [M-(CO+H or CI]²⁺ ions.     

Investigation of uranyl nitrate ion pairs complexed with amide ligands using electrospray ionization ion trap mass spectrometry and density functional theory

Ion populations formed from electrospray of uranyl nitrate solutions containing different amides vary depending on ligand nucleophilicity and steric crowding at the metal center. The most abundant species were ion pair complexes having the general formula [UO(2)(NO(3))(amide)(n=2,3)](+); however, singly charged complexes containing the amide conjugate base and reduced uranyl UO(2)(+) were also formed as were several doubly charged species. The formamide experiment produced the greatest diversity of species resulting from weaker amide binding, leading to dissociation and subsequent solvent coordination or metal reduction. Experiments using methyl formamide, dimethyl formamide, acetamide, and methyl acetamide produced ion pair and doubly charged complexes that were more abundant and less abundant complexes containing solvent or reduced uranyl. This pattern is reversed in the dimethylacetamide experiment, which displayed lower abundance doubly charged complexes, but augmented reduced uranyl complexes. DFT investigations of the tris-amide ion pair complexes showed that interligand repulsion distorts the amide ligands out of the uranyl equatorial plane and that complex stabilities do not increase with increasing amide nucleophilicity. Elimination of an amide ligand largely relieves the interligand repulsion, and the remaining amide ligands become closely aligned with the equatorial plane in the structures of the bis-amide ligands. The studies show that the phenomenological distribution of coordination complexes in a metal-ligand electrospray experiment is a function of both ligand nucleophilicity and interligand repulsion and that the latter factor begins exerting influence even in the case of relatively small ligands like the substituted methyl-formamide and methyl-acetamide ligands.     

Adiabatic hyperspherical study of three-particle systems

In an effort to unravel the dependence of the dynamics of three-particle systems on the relative mass of the particles, an adiabatic treatment in hyperspherical coordinates is used to study the ground state and some doubly excited states of several three-particle systems. Surprisingly, the binding energy of the ground state and of several doubly excited states exhibits a nearly linear dependence on the atomic reduced mass.     

聚乙烯醇载体的研究 Ⅲ.由聚乙烯醇衍生的活性酰胺和活性酯及其作为亲合色谱载体的应用

<正> 亲合色谱法在现代生物学和医学上作为新的得力分离分析手段而方兴未艾。合色谱法是将相互在生物学上具有特异亲合力的某一化合物作为固定相接在水不溶性载体上,利用对此固定相的亲合力的差异以分离提纯另一化合物为目的的方法。常用的水不溶性载体如天然纤维素,葡聚糖凝胶,琼脂糖凝胶等或因成型不易,或因对酸,霉菌等的不     

Die Addition von Imidazolderivaten an DCCI; eine Nebenreaktion bei der Synthese von Histidinpeptiden

In the synthesis of histidine peptides by means of the carbodiimide method the formation of N^im-amidino-histidine derivatives as secondary products has been observed. The reaction of imidazoles with DCCI leading to such compounds and their cleavage have been investigated. The significance of this side-reaction in peptide synthesis is discussed.     

Selected configuration interaction with truncation energy error and application to the Ne atom

Selected configuration interaction (SCI) for atomic and molecular electronic structure calculations is reformulated in a general framework encompassing all CI methods. The linked cluster expansion is used as an intermediate device to approximate CI coefficients B(K) of disconnected configurations (those that can be expressed as products of combinations of singly and doubly excited ones) in terms of CI coefficients of lower-excited configurations where each K is a linear combination of configuration-state-functions (CSFs) over all degenerate elements of K. Disconnected configurations up to sextuply excited ones are selected by Brown's energy formula, Delta E(K) = (E-H(KK))B(K)2/(1-B(K)2), with B(K) determined from coefficients of singly and doubly excited configurations. The truncation energy error from disconnected configurations, Delta E(dis), is approximated by the sum of Delta E(K)s of all discarded Ks. The remaining (connected) configurations are selected by thresholds based on natural orbital concepts. Given a model CI space M, a usual upper bound E(S) is computed by CI in a selected space S, and E(M) = E(S) + Delta E(dis) + delta E, where delta E is a residual error which can be calculated by well-defined sensitivity analyses. An SCI calculation on Ne ground state featuring 1077 orbitals is presented. Convergence to within near spectroscopic accuracy (0.5 cm(-1)) is achieved in a model space M of 1.4 x 10(9) CSFs (1.1 x 10(12) determinants) containing up to quadruply excited CSFs. Accurate energy contributions of quintuples and sextuples in a model space of 6.5 x 10(12) CSFs are obtained. The impact of SCI on various orbital methods is discussed. Since Delta E(dis) can readily be calculated for very large basis sets without the need of a CI calculation, it can be used to estimate the orbital basis incompleteness error. A method for precise and efficient evaluation of E(S) is taken up in a companion paper.     

Bonded tableau unitary group approach to the many-electron correlation problem

A spin-free symmetry-adapted valence bond (VB ) state, named bonded tableau (BT ), is deduced from the classical bonded function and labeled by an at most two-column Weyl tableau. The complete set, which is composed of the BT basis or canonical bonded tableau (CBT ), can be constructed from an overcomplete set of BT states. CI CBT and VB CBT are two kinds of complete sets that are constructed in this paper. They can be used, respectively, in the CI and VB theory. It is shown that there is a one-to-one correspondence between the labeling scheme for CI CBT and the Gelfand–Tsetlin (GT ) basis. This relationship enables an efficient generation and compact representation of the BT basis if one desires to use the known global representation scheme for the GT basis. Effective algorithms for the matrix element evaluation of unitary group generators and products of generators between BT states are presented. In the formulation, the action of a generator on a BT state yields another BT state times a coefficient, so that the matrix elements of an arbitrary multiple product of generators are reduced to a calculation of the overlaps between BT states. The evaluation of the overlaps leads to a simple factorization into cycle contributions, whose values are given explicitly and only depend on the length parameters of the cycles. It is hoped that the presented formalism can facilitate the procedures for handling of the many-electron correlation problem.     

Peptides—XXXVII: Synthesis of the 1–37 fragment of a lysozyme analogue

Combination of the protected peptide fragments 1–16, 17–26 and 27–37 to yield the 1–37 portion of a lysozyme analogue is described. The fragments were combined using DCCI with the addition of HONSu, and the products purified mainly by gel filtration.