Bernard Kirtman | University of California, Santa Barbara (original) (raw)

Papers by Bernard Kirtman

Solid State Nuclear Magnetic Resonance, Sep 1, 2019

Cross Effect (CE) Dynamic Nuclear Polarization (DNP) relies on the dipolar (D) and exchange (J) c... more Cross Effect (CE) Dynamic Nuclear Polarization (DNP) relies on the dipolar (D) and exchange (J) coupling interaction between two electron spins. Until recently only the electron spin D coupling was explicitly included in quantifying the DNP mechanism. Recent literature discusses the potential role of J coupling in DNP, but does not provide an account of the distribution and source of electron spin J coupling of commonly used biradicals in DNP. In this study, we quantified the distribution of electron spin J coupling in AMUPol and TOTAPol biradicals using a combination of continuous wave (CW) X-band electron paramagnetic resonance (EPR) lineshape analysis in a series of solvents and at variable temperatures in solution-a state to be vitrified for DNP. We found that both radicals show a temperature dependent distribution of J couplings, and the source of this distribution to be conformational dynamics. To qualify this conformational dependence of J coupling in both molecules we carry out "Broken Symmetry" DFT calculations which show that the biradical rotamer distribution can account for a large distribution of J couplings, with the magnitude of J coupling directly depending on the relative orientation of the electron spin pair. We demonstrate that the electron spin J couplings in both AMUPol and TOTAPol span a much wider distribution than suggested in the literature. We affirm the importance of electron spin J coupling for DNP with density matrix simulations of DNP in Liouville space and under magic angle spinning, showcasing that a rotamer with high J coupling and "optimum" relative g-tensor orientation can significantly boost the DNP performance compared to random orientations of the electron spin pair. We conclude that moderate electron spin J coupling above a threshold value can facilitate DNP enhancements.

Brazilian Journal of Physics, 1994

Due to their intrinsic orientational anisotropy and elevated polarizability n-conjugated polymers... more Due to their intrinsic orientational anisotropy and elevated polarizability n-conjugated polymers are widely perceived as promising materials for use in nonlinear optical devices. Advances in the development of practical systems have been impeded, however, by the difficulties in developing a realistic theoretical description of the experimental situation. In this paper we present several examples of theoretical progress towards this goal and discuss how the different effects contributing to the polarization response in the systems can be quanti fied.

1^{1}1Bernard Kirtman, J. Chem. Phys., 41, 3262 (1964).""Author Institution: Department... more $^{1}$Bernard Kirtman, J. Chem. Phys., 41, 3262 (1964).""Author Institution: Department of Chemistry, University of CaliforniaThe barrier to internal rotation and its derivatives (derivatives of the barrier with respect to internal symmetry coordinates) were calculated for CH3SiH3CH_{3}SiH_{3}CH3SiH3 using small basis sets of Slater orbitals with the Hartree-Fock method and also were dettermined from microwave and infrared spectra.1spectra.^{1}spectra.1. The computed barrier of 1.98 kcal/mole and its A1A_{1}A1 symmetry derivatives: ---5.57 kcal/\AA mole for the C-Si stretch; ---1.26 kcal/rad. mole for the Si-C-H bend; and ---2.14 kcal/rad. mole for the Si-C-H bend show very good agreement with the experimental values of 1.67 kcal/mole for the barrier and ---5.42 kcal/\AA mole; ---1.16 kcal/rad. mole; and ---1.55 kcal/rad. mole for the three derivatives. The addition of dorbitals on silicon had no significant effect on the calculated results. The barrier derivatives with respect to the C-H and Si-H stretches were assumed to be negligible in the experimental analysis. The change in geometry as the molecule internally rotates and several force constants were also determined both theoretically and experimentally

INTERNATIONAL CONFERENCE OF COMPUTATIONAL METHODS IN SCIENCES AND ENGINEERING 2009: (ICCMSE 2009), 2012

Apart from zero-point averaging, there are vibrational contributions to second- and higher-order ... more Apart from zero-point averaging, there are vibrational contributions to second- and higher-order 'electronic' properties that can be related to the shift in equilibrium geometry induced by a perturbing field. These contributions often exceed the pure electronic term in systems of technological interest. Their computation by means of the finite fieldnuclear relaxation (FF-NR) method, and its several extensions, is described using the example of static and dynamic nonlinear optical properties. Mechanical and other anharmonicities, dependent upon the property, may be quite important. We discuss the evolving treatment of systems where such is the case. Special handling for electric (and/or orbital magnetic) properties is required for polymers, surfaces and solids when they are modeled as periodic in one or more dimensions. The current status of developments in this area is included along with speculation regarding ultimate application to nanosystems.

Journal of Chemical Theory and Computation, Dec 2, 2019

We present a new coupled Hartree-Fock(HF)/Kohn-Sham DFT perturbation method that accounts for the... more We present a new coupled Hartree-Fock(HF)/Kohn-Sham DFT perturbation method that accounts for the effect of enlarging the basis set in electronic structure calculations. In contrast with previous approaches, our dual basis set treatment yields not only a correction for the total energy, but also for the orbital eigenvalues and density. The zero-th order solution is obtained from the projection of the small basis set coefficients. Diagonalization of the full Fock matrix in the large basis set is avoided. In this first paper of a series, we develop the theoretical foundations of our approach for molecules, including the coupled-perturbed equations through second order and the energy expressions through fourth order-as our method complies with Wigner's 2n + 1 rule. The first-order perturbation equation turns out to be uncoupled and odd-order terms in the energy expansion vanish. In calculations on simple molecules, our method recovers over 93% (84%) of the missing DFT(HF) energy when going from the cc-pVDZ to the aug-cc-pVDZ basis,

Physical Chemistry Chemical Physics, 2010

We report periodic B3LYP density functional theory calculations for three-dimensional (3D) trans-... more We report periodic B3LYP density functional theory calculations for three-dimensional (3D) trans-polyacetylene (t-PA) fibers. Empirical dispersion terms, as proposed by Grimme, are included with an appropriate re-scaling to yield the B3LYP+D* method implemented in CRYSTAL06. The dispersion corrections are critical for obtaining correct unit cell parameters. In our calculations the out-of-phase P2(1)/n structure turns out to be a transition state for the interchain relative translational motion, which lies about 0.35 kcal mol(-1) above the two symmetrically located in-phase P2(1)/a minima. These results provide a possible new explanation for the observed XRD intensities. Our calculations should also be useful for comparison with more costly non-empirical treatments of 3D PA and other pi-conjugated polymers.

arXiv (Cornell University), Feb 13, 2023

arXiv (Cornell University), Feb 11, 2023

Solid State Communications, Apr 1, 2010

The electronic structure and effect of vanadium doping of BN nanowires are studied by first princ... more The electronic structure and effect of vanadium doping of BN nanowires are studied by first principles calculations. For the pure nanowires, it can be found that B atoms move inwards whereas N atoms move outwards, and BN nanowires have a constant band gap about 4.08 eV with larger diameter. The above-mentioned features are in agreement with those of BN nanotubes.

华南师范大学学报（自然科学版）, Nov 19, 2014

Springer eBooks, Nov 21, 2014

The elongation (ELG) method is a theoretical procedure for building up an arbitrary system by add... more The elongation (ELG) method is a theoretical procedure for building up an arbitrary system by adding small fragments, one by one, to an original and growing cluster. In this chapter the basic features of this method are elaborated by considering, first, the field-free problem at the single particle level. Of particular importance are the procedures for regional molecular orbital localization, as well as integral (and other) cut-offs, that act in concert to allow calculations to be restricted to an interactive region of essentially fixed size. In carrying out calculations there will sometimes be a limited number of delocalized molecular orbitals that cannot meet the localization criteria. A generalized technique for dealing with this circumstance, as well as two- and three-dimensional systems is described. The predicted overall linear scaling behavior is shown to be verified in practice. Finally, building upon the single particle case, we present our formulation of ELG-LMP2 for electron correlation and ELG-LCIS for excited electronic states.

Springer eBooks, 2017

The response to electrostatic fields by finite systems, which contain a large central region of c... more The response to electrostatic fields by finite systems, which contain a large central region of chemically identical building blocks, is shown not to depend on the boundaries of the system. Consequently, we can determine this response from that of the corresponding infinite periodic system and a vector potential theoretical-computational approach is presented for doing so. Both the electronic and structural response can be obtained with our approach. In many cases, neglecting the structural response is a poor approximation. To illustrate the performance of our treatment, extensive calculations have been carried out for simple model systems. Selected results are reported.

Advanced theory and simulations, Sep 30, 2018

It is demonstrated that there is an interconnection between surfaces, shapes, and bulk properties... more It is demonstrated that there is an interconnection between surfaces, shapes, and bulk properties of macroscopic, crystalline materials. Thus, the requirement that no atom in the bulk shall experience a force from charges associated with the surface, a requirement formulated in terms of a generalized Tasker condition, leads to an interplay involving transfer of charge between different surfaces for a given sample. Through this mechanism, so-called polar surfaces, often considered to be essentially unstable, can always be stabilized. Density functional theory slab calculations are carried out to obtain typical surface charges and associated bulk geometric displacements for an individual surface. Then the consequences of the interplay between surfaces are studied through one property that is usually considered a bulk property, namely the polarization / dipole moment per volume, and one property that is usually related to an individual surface, that is, heterogeneous catalytic activity. Model calculations illustrate the importance of charge transfer between surfaces in both cases. These calculations provide a guide for much more difficult ab initio computations that might be carried out in the future.

Synthetic Metals, Apr 1, 1995

A semi-classical rnodel for the longitudinal linear and nonlinear polarizability of r-conjugated ... more A semi-classical rnodel for the longitudinal linear and nonlinear polarizability of r-conjugated polymers. based on the electrostatic potential, is present.ed. This model provides new insights regarding the response of the polymer to an external electric field. It, also leads to an efficient t,ransfer matrix method for extending calculat,ions of finite chains to t.he very large chain limit.

Journal of Physical Chemistry C, May 16, 2018

We study the interplay between surface and bulk properties of macroscopic materials. It is demons... more We study the interplay between surface and bulk properties of macroscopic materials. It is demonstrated that so-called polar surfaces may be stabilized through a charge redistribution between the complete set of surfaces that depends upon the overall shape of the sample and the nature of the material. This charge redistribution, in turn, is governed by certain constraints that we call generalized Tasker conditions. The same surface, but for samples of different shape, may have different surface charges. Besides its stabilizing effect, the charge redistribution also is shown to particularly affect the dipole moment per repeat unit, a bulk property. For the latter, it is established that essentially any physically meaningful value is possible (depending upon the shape and material), in contrast to the often made assumption that different samples of the same material will have values that differ by, at most, a lattice vector. Finally, some recent experimental and theoretical results for polar surfaces are discussed in terms of the analysis presented here. the surface through a charge redistribution. When the surface is subsequently exposed to water vapor a further stabilization occurs that we consider is likely accompanied by a further charge redistribution. A detailed analysis of this qualitative picture, however, lies outside the scope of the present work.

Journal of Chemical Physics, Apr 15, 1973

A simple, but accurate, unrestricted orbital product approximation for the spatial electronic wav... more A simple, but accurate, unrestricted orbital product approximation for the spatial electronic wavefunction of ordinary covalent molecules is developed. Each orbital consists of a pure atomic function which is rescaled in the molecule and, then, polarized by a variable effective internal field. Thus, the molecular orbitals are expressed in terms of known atomic polarization functions. Illustrative calculations on H2, LiH, and BeH+, utilizing single Slater-type orbitals as the undistorted atomic functions, give 50%–75% of the maximum possible energy improvement over the restricted Hartree-Fock model. It is shown how our distorted-atoms-in-molecules (DAM) approximation can be employed together with the distinguishable electron perturbation method to determine first-and second-order physical properties to within about 10%. We also express the hope that the error in computed electron correlation energies can be reduced to less than 1 kcal/mole per electron pair. Of course, DAM may be used directly to estimate physical properties with medium (∼ 40%) accuracy.

Acs Symposium Series, May 5, 1996

Journal of Chemical Physics, Jul 15, 1983

The density matrix treatment of localized electronic interactions in large systems is extended to... more The density matrix treatment of localized electronic interactions in large systems is extended to separated electron pair wave functions. We make a local space approximation and carry out a Newton–Raphson linearization which leads to effective SCF equations of relatively small dimensionality. Delocalization is included by rigorously satisfying the orthonormality condition on the natural orbitals as well as the normalization requirement on the pair functions. Both the orbitals and the pair coefficients are fully optimized.