Excited-state Moments Research Articles

Dipole Moments and Second-Order Hyperpolarizabilities of Imidazole-Based Betaines Determined from the Solvatochromic Effect

Abstract We report on results of measurements of the solvatochromic effect in two benzimidazole-based betaines (hereafter referred to as IB1 and IB2). The solvatochromic shifts exceed 2000 cm−1 in both molecules; however, the directions of the shift were found opposite in spite of similar chemical structures of the molecules under study. The ground-state dipole moments in both betaines exceed 13 Debye units, whereas the excited-state moments, calculated from the solvatochromic shift, differ markedly amounting to ca. 3 D in IB1 and 18 D in IB2. Quantum-chemical calculations indicate that betaines enjoy some freedom of orientation of their fragments which may influence the energies and intensities of the electronic transitions. The second-order hyperpolarizabilities, estimated from the measurements amount to ca. 20×10−40 m4/V (4.8×10−30 esu) for IB1 and 55×10−40 m4/V (13.8×10−30 esu) for IB2.

ELECTRONIC STATES OF THE INDOLE-ACRYLAMIDE MOLECULAR PAIR

A model is suggested for tryptophan fluorescence quenching by acrylamide based on the prediction that acrylamide can absorb a photon from the excited indole moiety and then dissipate the optical energy into a sink of fast exchanging conformations. Semiempirical electronic structure calculations of the indole-acrylamide pair indicate little actual intermolecular orbital mixing at van der Waals contact distances. However, the two lowest singlet transitions of the molecular pair, assigned to the acrylamide (pi *)----n(O) line and to the indole 1Lb----1A1 line, respectively, vary significantly in energies and in transition and excited state moments with the geometry of interaction between the two entities. The distribution of optimal quenching coordinations depends separately on the benzene and pyrrole portions and has a distinctly non-spherical shape at these distances.

A new method for the determination of the dipole moment of an electronic excited state: The π of cyclohexanone and the 1La of indole

AbstractIn this work, a new method for the determination of a Franck‐Condon excited state moment in solution is described. This method is based both on the study of the solvatochromic effect and on the use of Onsager reaction field theory. The method we propose requires (1) calorimetric measurements of the heat of transfer from solvent S1 to solvent S2 for the solute in its ground state and for a homomorph; (2) the absorption frequencies of the solute in the two solvents and (3) experimental determination of the solute and homomorph ground state dipole moments.It is easy to show that a very simple relationship exists between these various experimental quantities if the two solvents are such that the reaction field theory can be applied to describe the dipolar part of the solvent solute interactions. The use of the homomorph method permits one to take into account the variation of the cavity term from solvent to solvent and the variation of the solute solvent dispersive energy term with the nature of the solvent. Moreover, the method does not requires neither the estimation of the cavity radius nor the dangerous assumption that the dipole is centred in the cavity. This new method, which compared to older methods, must give more reliable results, is applied to the π* state of cyclohexanone and to the 1La state of indole.

Ground and excited state dipole moments of all-t r a n s-retinal and all-t r a n s-retinylidene-n-butylamine in solution

Ground state dipole moments of all-trans-retinal and all-trans-retinylidene–n-butylamine have been determined by dielectric constant measurements of dilute hexane solutions. Excited state dipole moments for the intense low energy absorption band have been estimated from the ground state moments and an analysis of the solvent shifts of absorption spectra using a corrected solvent shift theory. The ground state moments of all-trans-retinal and the n-butylamine Schiff’s base were found to be 4.6 and 2.1 D, respectively. The corresponding excited state moments are approximately 6 and 5 D.

Electronic spectra and magnetic circular dichroism of the uranyl ion. Part II. The effect of reduced symmetry on the spectra

We have examined the absorption and m.c.d. spectra of the dinitrate, trinitrate, and tetranitrate complexes of the uranyl ion and have interpreted the results in terms of an axial symmetry perturbation on the D∞h states characterised in Part I. It is found that this gives a consistent account of the m.c.d. spectra at liquid-helium temperatures. We have also investigated the striking changes in structure observed in the absorption spectrum of the dinitrate complex in various solvents and have obtained quantitative estimates of reduction in symmetry from changes in the excited state moment.

Excited-State Dipole Moments, Polarizabilities, and Local Fields in Molecular Crystals; Polarizabilities of Benzophenone in its Lowest Singlet and Triplet nπ Excited States

A refined calculation that accounts for molecular polarizabilities, crystal reaction fields and resulting field induced moments in polar molecular crystals allows excited-state dipole moments and polarizabilities to be derived from optically determined Stark splitting versus applied electric field results. The determination of these excited-state properties in a dipolar lattice is particularly dependent on the evaluation of the local field at a molecular site. For this case, the local field is evaluated by introduction of an ellipsoidal cavity approximation and by using the Ewald-Kornfeld lattice summation method for dipolar arrays. The dipolar benzophenone crystal was chosen to demonstrate this method. The dipole moments of the lowest 1nπ* and 3nπ* states of benzophenone have been determined and are 1.23 D and 1.72 D, respectively; these are the zero field values and refer to the benzophenone molecules in the absence of the crystal environment. The polarizabilities of benzophenone in its ground, 1nπ* and 3nπ* states are, in units of 10−23 cm3, 0.532, 0.898, and 0.56, respectively. The excited-state moments are nearly parallel to the ground state moment in the crystal environment and the benzophenone dipole does not reverse direction in going from the ground to the lowest singlet and triplet states.

5142-Å Transition in Thiophosgene (π* ← n); Rotational Analysis and Excited-State Structure

Rotational structure is observed in the 210310410 vibronic band of the lowest singlet π* ← n transition of thiophosgene (5142 Å). Computer analysis gives three excited-state moments of inertia, from which, assuming a value for the C–S bond length, an excited-state structure may be determined.

Electric-Field-Induced Spectra: Theory and Experimental Study of Formaldehyde

An experiment is described which measures the modulation, produced by an applied electric field, of the absorption coefficient of the rovibronic spectrum of a gaseous sample. Phase-sensitive detection provides for measurement of 0.01% modulation at a spectral resolution of 0.1 cm− 1 in the near ultraviolet. A theoretical development of the electric-field-induced spectral line shape and intensity as an expansion in powers of the applied field is presented. The important molecular parameters are the ground-state and excited-state dipole moments and rotational constants; measurement of the excited-state moment is discussed. The dependence of line intensity on quantum numbers is predicted to be such as to emphasize transitions to states of low angular momentum; observation of this emphasis is of importance in assigning spectra. The correspondence between field-induced and absorption spectra in a single experiment lends great confidence to assignments of specific absorption features. Experimental study of formaldehyde confirms many of the predictions of the theory. Assignments are obtained for the low-K subbands in the 3390-, 3430-, and 3520- Å bands, and the asymmetry parameters are presented. Effects of asymmetry on the EFS are shown. A method of determination of the excited-state dipole moment based on measurement of ratios of intensities of EFS in different polarizations is presented. The dipole moment of HCHO in the A 1A″ state is determined to be 1.4 ± 0.1 D.

A M ssbauer study of hyperfine interactions in dysprosium metal

Nuclear hyperfine interactions in Dy metal have been investigated in a temperature range spanning the ferromagnetic, antiferromagnetic and paramagnetic regions (4 2-300 °K), using the Mossbauer effect in 161Dy. The effect was observed in both the 25 6 kev and 74 5 kev γ-ray transitions. No detectable changes in the hyperfine spectra were observed in traversing the ferromagnetic-antiferromagnetic transition. The temperature variations of the effective magnetic field and the electric field gradient at the Dy nucleus are shown to be in fair agreement with a free-ion theory. The ratios of excited-state moments to ground-state moments for both transitions were also measured. For the 25 6 kev transition, values of -1 21±0 02 and +1 01±0 03 were obtained for the magnetic dipole and electric quadrupole moments respectively, and for the 74 5 kev transition similar ratios of +0 84±0 05 and +0 48±0 04 were obtained. Using independently measured values for the ground-state moments we have thus estimated the excited-state moments and compared them with values predicted by nuclear theory.

The Asymmetric Rotor. IX. The Heavy Water Bands at 2787 cm−1 and 5373 cm−1

The combination band (110) of the two stretching fundamentals of D2O is reported and analyzed to yield ν0=5373.2 cm−1 and the excited state moments of inertia 1.910, 3.931, and 5.929×10−40 g cm2. The same method of analysis applied to the unsymmetrical fundamental band (100) envelope gives ν0=2787.5 cm−1 and the excited state moments 1.881, 3.876, and 5.843×10−40 g cm2.