Silvano Romano

Merging and bifurcation of 4+2 and 2+4 cycloaddition modes in the archetypal dimerization of butadiene. A case of competing bispericyclic, pericyclic and diradical paths

The dimerization of butadiene has been explored by using DFT methods at the B3LYP level with the 6-311+G** basis set. A concerted bispericyclic TS for the endo pathway and a concerted pericyclic TS for the exo pathway are the lowest passes for the dimerization and occur at almost the same energy thus accounting for the lack of stereochemical preferences in the dimerization. Diradical paths involving two unswitched transoid butadiene moieties are competing and account for the formation of minor amounts of trans-1,2-divinyl cyclobutane and 1,5-cycloctadiene.

Merging of 4+2 and 2+4 cycloaddition paths in the regiospecific dimerization of methacrolein. A case of concerted crypto-diradical cycloaddition

Abstract The regiospecific dimerization of methacrolein takes place through a very asynchronous and symmetrical transition structure, which shows a merging of the 4+2 and 2+4 cycloaddition paths. The geometrical features of the transition structures correspond well to a case of diradical formation. A cage of secondary orbital interactions restricts the flexibility of the diradical transition structure and stabilizes it.

Computer simulation studies of anisotropic systems. XXVI. Liquid crystal dimers: A generic model

A generic model has been proposed with which to understand the unusual properties of liquid crystal dimers, in particular their behavior at the nematic–isotropic transition. In the model the many conformational states available to real dimers are replaced by just two, a linear and a bent conformer which are allowed to interconvert. The nematic behavior of the model has been explored with the aid of the molecular field approximation and found to be in good qualitative agreement with that of real dimers, thus providing a valuable aid to our understanding of them. To see to what extent the use of the molecular field approximation influences the predictions we have undertaken a Monte Carlo computer simulation study of the generic model. To facilitate the simulations the particles have been confined to the sites of a simple cubic lattice with the anisotropic interactions being restricted to nearest neighbors and having the form −(e/4)P2(cos β), where β is the angle between the anisometric groups. The other adv...

The Remarkable Cis Effect in the Ene Reactions of Nitrosocarbonyl Intermediates

Nitrosocarbonyls are fleeting and highly reactive intermediates that undergo ene reactions in a two-step fashion. The addition steps are rate and product determining and lead to polarized diradicals that readily enter the H-abstraction step yielding the ene products. The addition TSs are reached early, and the stabilizing CH...O contacts drive the reactions to the cis adducts. B3LYP calculations alone do not describe the correct ordering of addition TSs in the ene reaction with trimethylethylene and (E)- and (Z)-3-methyl-2-pentenes. Only at the MPWB1K level of treatment, medium-range noncovalent interactions are successfully recovered, accounting satisfactorily for the experimental selectivities. The more stable and isolable ArNOs exhibit late addition TSs, and distortion energies become dominant driving the reaction exclusively to the Markovnikov adducts.

Orientational ordering transition in a continuous-spin ferrofluid

We have considered a classical system, consisting of particles with centre-of-mass coordinates x∈Rd and equipped with a one-component spin σx∈R1; their pairwise additive interaction potential is defined by W(x,σx,y,σy)=Φ(∥x−y∥)−J(∥x−y∥)σxσy, where Φ(∥x−y∥) contains a hard-core term and J(∥x−y∥)⩾0. We prove the existence of orientational long-range order (magnetization) for large chemical potentials and low temperatures, when d⩾2; the same result holds for d=1 and long-range ferromagnetic interactions. Our general model can be realized by extremely anisotropic interaction potentials involving continuous multi-component spins, where only one component is involved in the coupling.

On the xy model and its generalizations

Abstract The simplest plane rotator model consists of two-component unit vectors, associated with a D-dimensional lattice (say Z D ), parameterized by polar angles ϕj, and interacting via a ferromagnetic potential restricted to nearest neighbours U=Ujk=−ϵcos(ϕj−ϕk); here ϵ is a positive quantity setting energy and temperature scales (i.e., T ∗ =k B T/ϵ ). On the other hand, the xy model involves three-component unit vectors, parameterized by polar angles (θk,φk), but with two components only coupled by the interaction, i.e., V=Vjk=−ϵsinθjsinθkcos(φj−φk). We consider here generalized xy models, where the interaction is defined by W=Wjk=−ϵ(sinθjsinθk)Lcos(φj−φk), L∈ N , L⩾1, and point out rigorous comparison inequalities holding for them; in turn, these can be used to prove existence and type of phase transitions when D⩾2; Mean Field and Two-Site Cluster estimates of the ordering transition temperatures are calculated as well, for D=3 and L⩽4.

On long-range order in low-dimensional lattice-gas models of nematic liquid crystals

Abstract The problem of the orientational ordering transition for lattice-gas models of liquid crystals is discussed in the low-dimensional case d = 1, 2. For isotropic short-range interactions, orientational long-range order at finite temperature is excluded for any packing of molecules on the lattice Z d ; on the other hand, for reflection-positive long-range isotropic interactions, we prove the existence of an orientational ordering transition for high packing ( μ > μ 0 ) and low temperatures ( β > β c ( μ )).

Effect of electric field and temperature gradient on the orientational dynamics of liquid crystals in a microvolume cylindrical cavity

We have considered a homogeneously aligned liquid crystal (HALC) microvolume confined between two infinitely long horizontal coaxial cylinders and investigated dynamic field pumping, i.e., studied the interactions between director, velocity, and electric E fields as well as a radially applied temperature gradient inverted Delta T, where the inner cylinder is kept at a lower temperature than the outer one. In order to elucidate the role of inverted Delta T in producing hydrodynamic flow u, we have carried out a numerical study of a system of hydrodynamic equations including director reorientation, fluid flow, and temperature redistribution across the HALC cavity. Calculations show that only under the influence of inverted Delta T does the initially quiescent HALC sample settle down to a stationary flow regime with horizontal component of velocity u(eq)(r). The effects of inverted Delta T and of the size of the HALC cavity on magnitude and direction of u(eq)(r) have been investigated for a number of hydrodynamic regimes. Calculations also showed that E influences only the director redistribution across the HALC but not the magnitude of the velocity u(eq)(r).

Computer simulation study of a two-dimensional nematogenic lattice model based on the Nehring-Saupe interaction potential

Abstract We have considered a nematogenic lattice model, consisting of 3-component unit vectors, associated with a 2-dimensional lattice, and interacting via the nearest-neighbour potential model proposed by Nehring and Saupe [Int. J. Mod. Phys. 54 (1971) 337], and already studied by simulation in 3 dimensions [Int. J. Mod. Phys. 13 (1999) 3879; Mod. Phys. Lett. B 15 (2001) 137]. The model is defined by Ψ jk =ϵ − 3 2 (3a j a k −τ jk ) 2 +1 , r = x j − x k , s = r /| r |, a j = u j · s , a k = u k · s , τ jk = u j · u k . Here the 2-component vectors x j ∈Z 2 define centre-of-mass coordinates of the particles, and u k are three-component unit vectors defining their orientations; ϵ is a positive quantity setting energy and temperature scales (i.e., T ∗ =k B T/ϵ ); this model can be regarded as the anisotropic counterpart to the generic Lebwohl–Lasher lattice model; in 2 dimensions, its anisotropic character does not preclude the existence of orientational order at finite temperature. The model produces a ground state where particles are aligned along a lattice axis; both Mean Field (MF) predictions and simulation results for the second-rank ordering tensor show a low-temperature regime where the system becomes biaxial, with the main director aligned along a lattice axis; at higher temperature there is a transition to uniaxial order with negative order parameter, and director orthogonal to the lattice plane; this orientational order survives up to temperatures higher than the transition temperature of the 3-dimensional counterpart, possibly at all finite temperatures. MF predictions and simulation results appear to agree qualitatively, but in quantitative terms the MF prediction for the transition temperature is some 45% too high.

First-order transitions for some generalized XY models

The present invention relates in part to hWART nucleic acid molecules. The invention also relates in part to nucleic acid molecules encoding portions of hWART full-length proteins, nucleic acid vectors containing hWART nucleic acid molecules, recombinant cells containing such nucleic acid vectors, polypeptides purified from such recombinant cells, antibodies to such polypeptides, and methods of identifying compounds that modulate the function of an hWART polypeptide. Also disclosed are methods for diagnosing abnormal cell proliferative conditions in an organism using hWART-related molecules or compounds.