Screened dipolar interactions in some molecular crystals

Munn, R. W.; Hurst, Michael O.

doi:10.1016/0301-0104(90)85018-r

Cited by 19 publications

(5 citation statements)

References 28 publications

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“…There appears to have existed a notion (recently refuted by a statistical analysis) that polar molecules tend to crystallize in centrosymmetric space groups. Munn and Hurst suggested earlier that this notion might be connected to a destabilizing surface energy in polar space groups. They criticized this idea, referring to “the experimental observation that polar molecular crystals do not usually exhibit a macroscopic dipole moment”.…”

Section: Discussionmentioning

confidence: 96%

Coulomb Energy of Polar Crystals

and

Kroon

1997

J. Phys. Chem. B

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A simple derivation is given to arrive at an expression for the Coulomb energy of a polar crystal with emphasis on its dependence on the external form of the crystal. It is found that the existing treatments are incomplete, since they refer to the central cell in the crystal, which may have an energy different from the average one. It is stressed that there is a very simple relation between results from calculations using a cutoff scheme and results from Ewald summation. Explicit formulas for the form-dependent contribution are developed for the case of a rectangular crystal. However, the usual approach in which that contribution is neglected appears to be a reasonable one for practical applications.

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Section: Discussionmentioning

confidence: 96%

Coulomb Energy of Polar Crystals

and

Kroon

1997

J. Phys. Chem. B

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“…Sigelle et al showed that POM retains the NLO properties while crystallizing in the noncentrosymmetric space group P 2 1 2 1 2 1 . The X-ray structure of POM had been determined earlier by Shiro et al, and a recent study by Munn and Hurst shows that the dipole sum in POM crystals nearly vanishes. Subsequently, the electrooptic (EO) coefficients of POM crystals were measured by Sigelle and Hierle, and Zyss et al also determined the acoustic and acoustooptic properties and studied the second harmonic and sum-frequency generation in POM …”

Section: Introductionmentioning

confidence: 76%

Electronic Structure Analysis of the Nonlinear Optical Materials 4-NitropyridineN-Oxide (NPO) and 3-Methyl-4-nitropyridineN-Oxide (POM)

Glaser

Chen

1997

Chem. Mater.

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The molecules 4-nitropyridine N-oxide (NPO) and 3-methyl-4-nitropyridine N-oxide (POM) and the models nitromethane and N-methylnitrone were studied with ab initio electronic structure theory at the RHF level and with the inclusion of electron correlation using perturbation and density functional theories. At the highest level, MP2(full)/6-311G**// MP2(full)/6-31G*, the dipole moments µ(NPO) ) 0.97 and µ(POM) ) 0.89 D were obtained. Methyl substitution leads to only a small reduction of ∆µ < 0.1 D and the computed dipole moments are in excellent agreement with recent experimental data. The dipole vector of NPO points away from the nitro group (-pole) toward the NO group (+pole) and the dipole vector in POM is rotated such as to point toward the Me-substituted half. The electric quadrupole moments of NPO and POM indicate quadrupolarity {-+ -} along all axes and the |Q zz | values are particularly large. Natural Population analysis reveals the common electronic motif for NPO and POM consisting in an electron-deficient hydrocarbon midsection embedded between electron-rich functional groups. The dipole direction in the pyridine N-oxides thus does not reflect contributions by the quinoid resonance form (electron density shifts from the NO to the NO 2 group) to the ground-state electronic structure. The directions of the molecular dipole moments of the pyridine N-oxides are the simple result of vector addition of the two inward pointing dipoles that are associated with the functional groups and caused by electronegativity differences. In contrast to X-ray electron density studies, the electronic consequences of H/Me replacement are found to be localized. Approximate "molecular dipole moments" based on point charge models (PCM) are compared to the correct dipole moments. The analysis of the PCM-derived dipole moments shows that a discussion of solid-state effects on the molecular dipole moments of NPO and POM must be postponed until the true dipole moments in the crystal have been more rigorously established.

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“…Permanent fields in molecular crystals are calculated to be of the order of 10 GV m Ϫ1 and may vary by 50% from the bulk into the surface layer. 12 These fields are large enough to produce effective dipole moments in the bulk crystal environment that are calculated to be as much as 50% different from those of the free molecules, 16,17 and precise electron density measurements backed up by ab initio molecular orbital calculations confirm that the effective dipole moment of MNA in the crystal is considerably enhanced over that in the gas phase. 18 These two mechanisms both concern the dielectric response of the crystal, and are treated by similar formalisms.…”

Section: Mechanisms Of Surface Shgmentioning

confidence: 94%

Microscopic theory of molecular crystal surface second-harmonic generation

Munn

1995

The Journal of Chemical Physics

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Articles you may be interested inMicroscopic calculation of surface-induced second-harmonic generation in crystals of para-nitroaniline Second-harmonic generation ͑SHG͒ is induced in molecular crystals by the variation through the surface region of the electric fields associated with the optical wave and with the permanent electric multipole moments of the molecules. It arises from the molecular first hyperpolarizability, whether permanent or induced by electric fields at noncentrosymmetric sites in the bulk crystal or at sites in the surface region where centrosymmetry is lost. Algebraic expressions suitable for iterative numerical evaluation are derived for the molecular response coefficients as affected by the multipole fields in the surface region. Detailed expressions are then derived for the surface second-harmonic generation in terms of the spatially varying molecular response and a microscopic expression that is derived for the variation of optical electric field through the surface region. The treatment does not require the introduction of a surface dielectric constant but incidentally yields a microscopic expression for it. The quadratic susceptibilities for surface SHG in anthracene and pyrene are roughly estimated as respectively 1/80 and 1/20 that in p-nitroaniline, reported to be 4 fm V Ϫ1 ; the coefficients could vary by one or two orders of magnitude among different surfaces.

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Screened dipolar interactions in some molecular crystals

Cited by 19 publications

References 28 publications

Coulomb Energy of Polar Crystals

Coulomb Energy of Polar Crystals

Electronic Structure Analysis of the Nonlinear Optical Materials 4-NitropyridineN-Oxide (NPO) and 3-Methyl-4-nitropyridineN-Oxide (POM)

Microscopic theory of molecular crystal surface second-harmonic generation

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