Configuration interaction calculations for the ground and excited states of ozone and its positive ion: Energy locations and transition probabilities

Large-scale configuration interaction calculations (including energy extrapolation) are reported for the various states of ozone and its positive ion. The first four dipole-forbidden electronic transitions in the O₃ spectrum are calculated to occur at 1.20, 1.44, 1.59, and 1.72 eV, respectively, while the corresponding low-energy-allowed species known as the Chappuis, Huggins, and Hartley bands are predicted to possess vertical excitation energies of 1.95, 3.60, and 4.97 eV, respectively. These results all appear to fit in quite well with the observed location of the pertinent spectral features, with respect to both energy and intensity. The 5- to 8-eV region of the ozone spectrum is found to be characterized by a series of double-excitation transitions out of the highest three occupied orbitals to the lowest unoccupied 2b₁(π^*) species. The strong features observed at 9.3 and 10.2 eV are thereupon calculated to result primarily from transitions into the 7a₁ (σ^*) MO (calculated 9.29 and 10.05 eV) and in the former case also from the 3s members of the various O₃ Rydberg series (calculated 9.21 and 9.38 eV). Finally the order of the first three ip's is found to be 6a₁, 4b₂, and 1a₂, while the feature in the neighborhood of 16 eV is attributed to a shake-up state of ²B₁ symmetry.     

Ab initio MRD-CI study of ethane: The 14–25 eV PES region and Rydberg states of positive ions

Ab initio calculations of the MRD-CI type are reported for various states of the C₂H₆⁺ ion in two different nuclear geometries and the results are compared with the experimentally observed ethane PES in the 14–25 eV region. The calculated vertical IP values for ionization out of the 1e_u, 2a_2u and 2a_1g MO's respectively agree well with the locations of the three ionization maxima in this spectral range. The analogous findings for excitation out of the relaxed ionic ground state find several relatively low-lying species which occupy a 2pσ* MO in addition to states resulting from simple ionization of the neutral molecule. A number of Rydberg states are also calculated at the relaxed-ion geometry, from which results it is determined that the quantum defects for such species are from 0.40–0.45 units smaller than for their counterparts in neutral systems; these findings are clearly consistent with a decrease in the core penetrability of the Rydberg electron as the effective charge is increased to Z = 2.     

An approach to developing a force field for molecular simulation of martensitic phase transitions between phases with subtle differences in energy and structure

d,l-Norleucine is one of only a few molecules whose crystals exhibit a martensitic or displacive-type phase transformation where the emerging phase shows a topotaxial relationship with the parent phase. The molecular mechanism for such phase transformations, particularly in molecular crystals, is not well understood. Crystalline phases that exhibit displacive phase transitions tend to be very similar in structure and energy. Consequently, the development of a force field for such phases is challenging as the phase behavior is determined by subtle differences in their lattice energies and entropies. We report an approach for developing a force field for such phases with an application to d,l-norleucine. The proposed procedure includes calculation of the phase diagram of the crystalline phases as a function of temperature to identify the best force field. d,l-Norleucine also presents an additional problem since in the solid state it exists as a zwitterion that is unstable in vacuo and therefore cannot be characterized using high-level ab initio calculations in the gas phase. However, a stable zwitterion could be obtained using Onsager's reaction-field continuum model for a solvent (SCRF) using both Hartree-Fock and density functional theory. A number of force fields and the various sets of partial charges obtained from the SCRF calculations were screened for their ability to reproduce the crystal structures of the two known phases, alpha and beta, of d,l-norleucine. Selected parameter sets were then employed in free energy minimizations to identify the best set on the basis of a correct prediction of the alpha-beta phase transition. The Williams' nonbonded parameters combined with partial charges from SCRF-Polarized Continuum Model calculation were found to reproduce the structures of the phases accurately and also maintained their stability in extended molecular dynamics simulations in the Parrinello-Rahman constant stress ensemble. Moreover, we were also able to successfully simulate the phase transformation of the beta- to the alpha-phase. The identified force field should enable detailed studies of the phase transformations exhibited by crystals of d,l-norleucine and hence enhance our understanding of martensitic-type transformations in molecular crystals.     

Total syntheses of (±)‐cis‐ and trans‐11‐methoxydeethyleburnamonine

The total syntheses of 11‐methoxydeethyleburnamonines ( 4 ) and ( 13 ) were carried out with use of 6‐methoxytryptophyl bromide ( 5 ) as starting material. Compound 5 was converted in three steps to trans‐ester 8 . Acid‐catalysed epimerization of 8 , lithium aluminum hydride reduction of the ester group, tosylation and substitution with cyanide gave the cis‐nitrile 12 . Acid‐induced cyclization of 12 yielded mainly (±)‐trans‐11‐methoxydeethyleburnamonine ( 13 ), whereas base‐induced cyclization gave (±)‐cis‐11‐methoxydeethyleburnamonine ( 4 ).     

Everywhere -repetitive sequences and Sturmian words

Local constraints on an infinite sequence that imply global regularity are of general interest in combinatorics on words. We consider this topic by studying everywhere α-repetitive sequences. Such a sequence is defined by the property that there exists an integer N≥2 such that every length-N factor has a repetition of order α as a prefix. If each repetition is of order strictly larger than α, then the sequence is called everywhere α⁺-repetitive. In both cases, the number of distinct minimal α-repetitions (or α⁺-repetitions) occurring in the sequence is finite.A natural question regarding global regularity is to determine the least number, denoted by M(α), of distinct minimalα-repetitions such that an α-repetitive sequence is not necessarily ultimately periodic. We call the everywhere α-repetitive sequences witnessing this property optimal. In this paper, we study optimal 2-repetitive sequences and optimal 2⁺-repetitive sequences, and show that Sturmian words belong to both classes. We also give a characterization of 2-repetitive sequences and solve the values of M(α) for 1≤α≤15/7.     

Struktur/wirkung-Beziehungen bei Geruchsstoffen mit dem Bicyclo[2.2.2.]octan-Gerüst

Structure-Activity Relationships of Odorants with a Bicyclo[2.2.2]octane System The synthesis of the olfactory interesting homonojigikualcohol 2 and of its epimer 10 leads to the keto alcohols 6a, b and 7a, b as intermediates. The latter have OH and C?O groups (an AH/B system) fixed at the rigid bicyclic nucleus at different distances (2.8 and 4.7 Å) and confirm Ohloff's rule for odoriferous properties which is based on this distance.     

Electronic structure of the radicals NF and NCl. III. The radiative lifetimes of the bΣ and aΔ states

The radiative lifetimes of the b¹Σ⁺ and a¹Δ states have been evaluated by perturbation expansions including X³Σ⁻, a¹Δ, b¹Σ⁺, 1^3,1Π, 2^3,1Π, 2³Σ⁻ and 2¹Σ⁺ states. All wavefunctions result from large MRD CI calculations. The b—X transition is dominated by the parallel transition moment; it is found to be much stronger than the a—X transition. The calculated radiative lifetimes of τ(¹Σ⁺)=18 ms, τ(¹Δ)=2.2 s for NF and τ(¹Σ⁺)=2.5–3.5 ms for NCl are in good accord with corresponding experimentally deduced values. The lifetime for the a¹Δ state in NCl is found to be τ(¹Δ)=1.1 s, ie. much longer than derived from a recent experiment. Its magnitude is consistent with the τ(b¹Σ⁺)/τ(a¹Δ) ratio of similar systems and with the decrease in lifetime from NF to NCl and is thus believed to be quite reliable. A detailed analysis of all contributions of the perturber states to the transition mechanism is made and comparison with the related data in SO, O₂ and S₂ is undertaken. The b-a transition probability dominated by the quadrupole transition is fairly constant in all the systems in the order of A = 0.013 (NF) - 0.0013 (S₂) s⁻¹.     

Complete assignment of 1H and 13C NMR data of pravastatin derivatives

The complete ¹H and ¹³C NMR data of 27 pravastatin derivatives are presented. Assignment was achieved by use of 1D and 2D NMR experiments (selective 1D NOE, COSY, NOESY, HSQC, HMBC). Copyright © 2008 John Wiley & Sons, Ltd.