Experimental Ground-State Dipole Moments

Experimental dipole moments can be obtained in several different ways. The first and most widely used approach is based on the measurement of dielectric constants. The second group of methods utilizes microwave spectroscopy and molecular beams (the Stark effect method, the molecular beam method, the electric resonance method, Raman spectroscopy, etc.). 

Many dipole moments available in the literature were obtained from the measurements of dielectric constants in the vapor phase or in diluted solutions of polar compounds in nonpolar solvents [24-31]. All these methods are based on Debye s statistical theory [35]. 

The Debye equation relating the dipole moment, fi, to the total polarization of a substance, P, has the form 

When the constants are expressed numerically and the total polarization (expression on the left-hand side) is plotted against T, the dipole moment is obtained from the slope, b  

A satisfactory range of temperatures (at least 100 K) is needed. In the above form, the Debye equation can be used for gas-phase determinations only and thus it is not applicable to most heterocyclic compounds. 

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In general, experimental methods for the determination of excited-state dipole moments are based on experimental ground-state dipole moments and a change of the position of a spectral band (in an electronic spectnnn) caused by an electric field which can be external (electro-... 

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... 

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... 

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). 

Dipole moments have been measured in the ground and excited states for three derivatives of compound (1) <83CPL58>. Ground-state dipole moments are shown in Figure 4. The ground-state dipole moment of the parent structure (1) has been found experimentally to be 4.87 x 10-30 C m (1.46 D) <81JPC3181>. 

The ground-state dipole moments of BPHTs were calculated by evaluating the vector sum of the it moment (computed by the Pariser-Parr-Pople (PPP) method) and the a moment (from cr-bond moments) [17], a separate set of dipole moment values being obtained by the parametric method 3 (PM3) ([25] and references therein) (Table 5). By comparison, the calculated ground-state dipole moment values were considerably lower than the experimental ones. In some cases, the agreement between the experimental and calculated ground-state dipole moments was rather poor [17]. 

In the calculation of the first excited singlet-state dipole moments, the a moment was assumed to be the same as in the ground state and only changes of the it moment were considered. The authors also found a relatively good agreement between the calculated excited-state dipole moments and the average values of the experimental excited-state dipole moments (Table 5) [17]. For comparison, another set of calculated excited-state dipole moments is presented in Table 5 [25,91,92]. 

Experimental and calculated ground-state dipole moments of selected pyrimidines and purines... 

Abs absoiption transitions of cytosine in water experimental data for keto-N3H and imino tautomers correspond to 3-methylcytosine and 3-methylcytidine, respectively, obtained in aqueous solution at pH 11 [279], °ground state dipole moments of keto-N3H, imino and enol tautomers at the HF/6-311G(d,p) level are 8.15, 5.12 and 3.34 Debye, respectively, scaled (scaling factor 0.72) excitation energies, Rydberg contamination. 

The experimental data reported in the Table for gas phase have been extracted from measurements in dioxane solution by applying the Onsager reaction field model to eliminate the solvent effect [37], By contrast, the cyclohexane experimental dipole moments have been obtained from those reported in Ref. [37] re-including the proper reaction field factors. Once recalled these facts, we note that the observed solvent-induced changes on both ground and excited state dipole moments are quantitatively reproduced by the calculations. 

EFISHG yields projections of the /3 tensor on the direction of the molecular dipole moment (z-axis). Hence a specific linear combination of elements is obtained and not a unique -value that is sufficient to characterize the molecular second-order NLO response. This is a serious limitation of the technique some components of /3 may be large but will not show up in the experimental results because their projection on the direction of the molecular ground-state dipole is zero. However, the use of polarized incident light with polarization directions parallel and perpendicular to the externally applied electric field allows the extraction of further information on the /3 tensor. For planar molecules conjugated in the yz plane, components with contributions of the X direction may be safely ignored. Two linear combinations, /3 and of tensorial elements may then be determined (Wortmann et al., 1993), (123) and (124) ... 

The two-state model is also applied to determine the first-order hyperpolarizabilities based on the experimental measurements of the spectroscopic quantities [103]. The ground state and the CT excited state dipole moments, excitation energy, as well as transition dipole moment (or oscillator strength) can be determined through the solvatochromic effect measurements. In particular, the first-order hyperpolarizability can be obtained in such a way by employing Eq. (4) or Eq. (6a). 

As an example, the experimental and calculated first excited singlet-state dipole moments for selected fluorescent quinazolines are presented in Table 3, with the ground-state values included for comparison [120], For most compounds, the agreement is good or at least acceptable. 

An example of solvent-induced solvatochromic shifts (calculated at a characteristic snapshot from the MD trajectory for each solvent) on different electronic excited states is shown in Fig. 5.5. Inspection of this plot reveals that the electronic states with the dipole moments that are larger than the dipoles in the ground state (shown as solid red curves in Fig. 5.5) become increasingly stabilized (red-shifted) in polar solvents. For example, l Ai, l Bi, 2 Bi states, which dipoles are larger than in the ground state dipole (7.7 Debye), demonstrate systematic red shifts upon solvation. The red shift increases in more polar solvents (in the order of c-hexane, dioxane, and water). The most dramatic red shift is experienced by the experimentally observed l Ai charge-transfer state with the (gas-phase) dipole moment of 12.9 D. It is quite intriguing that this state (the lowest red state in Fig. 5.5) is only the third lowest excited state in the gas phase but becomes the lowest excited state in water. On the... 

An alternative means for the experimental evaluation of //da is by means of electroabsorption spectroscopy. " The electric field dependence of the absorption spectrum can be used to obtain the difference in the ground state and excited state dipole moments, A/UdaI- This may be combined with the transition dipole moment (in Debyes), l gel = 16[Cda/(108 x 10 da)]. to obtain (neglecting vibronic and nonresonance overlap contributions), ... 

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