Dipole moments ground-state

In quantum-chemical calculations of dipole moments, each possible or expected structure is characterized by its inherent wave function, i r, and its inherent electronic configuration [11]. The dipole moment is then defined by eq. (10) [11], 

In principle, methods that can be used to calculate dipole moments are the various MO methods, which can be divided into three large groups emipirical, semiempirical, and ai initio. In contrast, VB methods are generally not suited for calculations of dipole moments because here the motion of electrons is completely synchronized. If, however, one uses additional ionic, 

In the various MO methods (HMO, SCF-MO, PPP, etc.), the dipole moment, which corresponds to a system of point electric charges (charge densities), is defined by the relation 

A significant advantage of calculated dipole moments is that they are vectors and thus give a direction of the dipole moment whereas experimental dipole moments are normally obtained without any defined direction. 

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The Wd estimated from the RASCI is compared with other theoretical calculations [46, 54] in Table V. The present DF estimate of Wd deviates by 21%(29%) from the SCF (RASSCF) estimate of Kozlov et al. [46] and by 17% from the semiempirical result of Kozlov and Labzovsky [54], while our RASCI result departs by 6% (3%) from the SCF-EO (RASSCF-EO) treatment of Kozlov et al. [46] and is in good agreement with the semi-empirical result of Kozlov and Labzovsky. At this juncture, we emphasize that our computed ground state dipole moment of BaF (p(. = 3.203 debyes) is also reasonably close to experiment p(. = 3.2 debyes (see Table 5 of Ref. 38). 

These features, made compatible with the previously detailed electronic requirements, are respectively a vanishing ground state dipole moment requirements in the case of POM (18), chirality in the case of MAP (19), chirality and hydrogen-bonding in the case of NPP (20). Characterization and growth of these materials will be detailed further on. 

Polar groups, such as carbonyl (C=0) and hydroxyl (OH), have a strong ground state dipole moment and show strong IR absorptions at characteristic frequencies. The IR spectrum can thus be used as a fingerprint of molecular... 

To illustrate these energy levels, Figure 2.1 shows formaldehyde as an example, with all the possible transitions. The n —> n transition deserves further attention upon excitation, an electron is removed from the oxygen atom and goes into the n orbital localized half on the carbon atom and half on the oxygen atom. The n-n excited state thus has a charge transfer character, as shown by an increase in the dipole moment of about 2 D with respect to the ground state dipole moment of C=0 (3 D). 

There have been no previous direct non-BO studies of the response of H2 and its isotopomers to electric fields. The ground-state dipole moment of HD has been determined experimentally by Nelson and Tabisz [81] to be 0.000345 a.u. There have been several theoretical studies of the dipole moment of HD, all within the BO approximation but including adiabatic corrections. The calculated values by Wolniewicz, 0.000329 [83], Ford and Browne, 0.000326 [82], and Thorson et al., 0.000334 [84], aU agree well with the experimental value, although they are all about 5% too small. This is an extremely difficult experiment to carry out, and because all theoretical studies agree on the value, it... 

Attention should be paid to the fact that the electron motion from a donor to an acceptor or within spread-conjugated ion-radicals generates measurable electromagnetic radiation. In other words, there is one specific way to reveal intramolecular electron motion or intermolecular electron transfer in charge-transfer complexes. To probe electron transfer in this manner, the only fundamental requirements are that the molecule can be oriented in the external magnetic field. This takes place most easily if the species considered has a ground state dipole moment. On photoexcitation, there... 

The second experimental measure of the orientation of the dipole moment in the ground state of the bases was made by Seibold and Labhart405 for uracil and thymine. The direction of the ground state dipole moment with respect to the transition moment in the lowest excited singlet state has been determined from the influence of an... 

Steer and coworkers203 have performed electrochromism studies on pyranthione (4H-pyran-4-thione) (38) and xanthione (44). They found that the magnitude of the change in the dipole moment upon excitation, A(i = /i(A2) — li(So) is close to 2 D. The transition dipole moment for the So —> S2 transition is parallel to the direction of the ground state dipole moment, i.e. along the C2 axes which contains the C=S bond. 

We limit our discussion to polarization and polarizability in a single dimension for pedagogical reasons). The zero field wave functions substituted in the eq 5 yield the ground state dipole moment of the molecule. 

This paper summarizes the theoretical analysis of some new molecules with methylsulfonyl group as the electron acceptor group, describes the syntheses of new stilbene and azobenzene systems, and presents the measurements of their optical spectra, ground-state dipole-moments, and molecular hyperpolarizability coefficients, p. We compare theoretical and experimental results and comment on the potential usefulness of these chromophores as components for NLO materials. The incorporation of sulfonyl-containing chromophores into polymers, and the NLO properties of the resulting materials, will be discussed in our forthcoming paper (9). 

We have performed a series of semiempirical quantum-mechanical calculations of the molecular hyperpolarzabilities using two different schemes the finite-field (FF), and the sum-over-state (SOS) methods. Under the FF method, the molecular ground state dipole moment fJ.g is calculated in the presence of a static electric field E. The tensor components of the molecular polarizability a and hyperpolarizability / are subsequently calculated by taking the appropriate first and second (finite-difference) derivatives of the ground state dipole moment with respect to the static field and using... 

We started the analysis of the sulfonyl group as an acceptor with the calculation of 4-methylsulfonylaniline (I), which is an analogue of p-nitroaniline (II), and their methylated derivatives (III, and IV, respectively). We have found that while the ground state dipole moments are comparable for the nitro and sulfone derivatives (Table I), the p coefficients are different in magnitude, and depend on the method of calculation (Table I). 

Since the hyperpolarizability of a given molecule is a function of the donor and the acceptor properties, and nature of the conjugation path between them, we turned to the biphenyl system and analyzed the 4-amino-4 -methylsulfonylbiphenyl (V). The calculated ground state dipole moment of this molecule is smaller than expected for such an increase in the distance between the donor and the acceptor. 

Our calculations predict only minor differences between the ground state dipole moments for molecules containing nitro electron acceptors versus those possessing methylsulfonyl. In contrast, the hyperpolarizabilities behave much differently, in that calculated J3Z for the aminonitrostilbenes is about twice that of the aminosulfonylstilbenes and the nitroanilines are more than 5 times more nonlinear than the sulfonylanilines. The hyperpolarizabilities appear to be very sensitive to the details of the electron donors-acceptor interaction and hence accentuate the differences in the a values for nitro and methylsulfonyl. 

The measurement of ground-state dipole moments may help to establish the validity of the theoretical calculations. We measured two representative compounds, X, and XI (see below), where diallyl derivatives were used to increase solubility in nonpolar solvents (e.g., CC14). 

We have also measured the ground-state dipole moment of DANS (VII), in CHCI3, and obtained the value 7.6 D. This, indeed, was an encouraging result since it further supported the credibility of our molecular design approach that was developed to speed up the search for new NLO materials. 

In this study we have described theoretical calculations, syntheses, optical spectra, ground-state dipole moment measurements, and measurements of molecular second-order hyperpolarizability coefficients (/J) for new stilbene and azobenzene derivatives containing a methylsulfonyl group as the electron acceptor. We have shown that theoretical calculations can be used to predict the ratio of molecular hyperpolarizabilities between similar compounds, and that these gas phase calculations underestimate /J, probably as a result of the valence basis set used in the calculations. 

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