First excited state

Second-order effects include experiments designed to clock chemical reactions, pioneered by Zewail and coworkers [25]. The experiments are shown schematically in figure Al.6.10. An initial 100-150 fs pulse moves population from the bound ground state to the dissociative first excited state in ICN. A second pulse, time delayed from the first then moves population from the first excited state to the second excited state, which is also dissociative. By noting the frequency of light absorbed from tlie second pulse, Zewail can estimate the distance between the two excited-state surfaces and thus infer the motion of the initially prepared wavepacket on the first excited state (figure Al.6.10 ). 

The experiment is illustrated in figure B2.5.9. The initial pump pulse generates a localized wavepacket in the first excited state of Nal, which evolves with time. The potential well in the state is the result of an avoided crossing with the ground state. Every time the wavepacket passes this region, part of it crosses to the lower surface before the remainder is reflected at the outer wall of the potential. The crossing leads to... 

In this chapter, we discussed the significance of the GP effect in chemical reactions, that is, the influence of the upper electronic state(s) on the reactive and nonreactive transition probabilities of the ground adiabatic state. In order to include this effect, the ordinary BO equations are extended either by using a HLH phase or by deriving them from first principles. Considering the HLH phase due to the presence of a conical intersection between the ground and the first excited state, the general fomi of the vector potential, hence the effective... 

The Symmetry Properties of Wave Fuitctioits of Li3 Electronically First-Excited State in < 3 Permutation Group... 

Figure 4. Relaxed triangular plot [68] of the Li3 first-excited state potential energy surface using hyperspherical coordinates. Contours are given by the expression E (eV) =-0.56-1-0.045(n — 1) with n = 2,3,. The dissociation limit indicated by the dense contouring implies...

Figure 9. Vibrational levels of the first-excited state Lis calculated without consideration (NGP) and with consideration (GP) of geometric phase effect [12].

To define the state yon want to calculate, you must specify the m u Itiplicity. A system with an even ii n m ber of electron s n sn ally has a closed-shell ground state with a multiplicity of I (a singlet). Asystem with an odd niim her of electrons (free radical) nsnally has a multiplicity of 2 (a doublet). The first excited state of a system with an even ii nm ber of electron s usually has a m n Itiplicity of 3 (a triplet). The states of a given m iiltiplicity have a spectrum of states —the lowest state of the given multiplicity, the next lowest state of the given multiplicity, and so on. 

Time-dependent calculations often result in obtaining a wave function that oscillates between the ground and first excited states. From this solution, it is possible to extract both these states. 

Sandstrom et al. (65) evaluated the Kj value for 4,5-dimethyl-A-4-thiazoline-2-thione (46) in water (Scheme 19) K-j= 10. A-4-Thiazoline-2-thiones are less basic in the first excited state (61) than in the ground state, so application of Forster s cycle suggests that the thione form is even more favored in the first excited state. Huckel molecular orbital (HMO) calculations suggest that electronic effects due to substitution in... 

A bathochromic shift of about 5 nm results for the 320-nm band when a methyl substituent is introduced either in the 4- or 5-posiiion, The reverse is observed when the methyl is attached to nitrogen (56). Solvent effects on this 320-nm band suggest that in the first excited state A-4-thiazoline-2-thione is less basic than in the ground state (61). Ultraviolet spectra of a large series of A-4-thiazoline-2-thiones have been reported (60. 73). 

Steric overcrowding associated with the interaction betw een the thiocarbonyl group and a bulky alkyl group gives a bathochromic shift. This has been interpretated as evidence for a smaller thiocarbonyl group" in the first excited state (73). 

A-4-Thiazoline-2-one and its derivatives absorb in the region of 247 nm [244 nm in cyclohexane (106)]. Thks band involves a transition (30. 102). From PPP calculations, the first excited state of... 

F. 1-26. (a) ir-Bond order of the C-S bonds in the ground state, (fc) ir-Bond order of the C-S bonds in the first excited state, (c) Free-valence number of the intermediate diradicaf. (Most probable bicyclic intermediate resulting from the ring closure of the diradicai. 

An example of an investigation of vibrational motion in a bound (excited) electronic state is in the B state of I2 (see Section 73.2). Figure 9.44 shows potential energy curves for three electronic state of I2, the ground state the first excited state B IIq+ and a higher... 

A kinetic scheme and a potential energy curve picture ia the ground state and the first excited state have been developed to explain photochemical trans—cis isomerization (80). Further iavestigations have concluded that the activation energy of photoisomerization amounts to about 20 kj / mol (4.8 kcal/mol) or less, and the potential barrier of the reaction back to the most stable trans-isomer is about 50—60 kJ/mol (3). 

This difference is due to the two lone pairs on the oxygen. Of the six valence electrons on the oxygen atom, two are involved in the double bond with the carbon, and the other four exist as two lone pairs. In Chapter 4, we ll examine the IR spectra for these two molecules. The orbitals suggest that we ll find very different frequencies for the two systems. In Chapter 9, we ll look at the transition to the first excited state in formaldehyde. ... 

They also noted a strong solvent dependence in the lowest energy band, which corresponds to the lowest excited state of this molecule. This suggests that the dipole moment will change sign as the molecule moves from its ground state to the first excited state. 

Perform a Cl-Singles study of this system, and compare your excited state results with this experimental data. In addition, calculate the dipole moment for the ground and first excited state. 

We ve specified five excited states with NSlotes=5 (the reasons will be clear in a moment). The DensitysAll keyword tells Gaussian to perform the population analysis using all available densities the SCF (ground state) density, the Cl one-partide density, and the Cl (Cl-Singles) density. The population analyses using excited state densities will be performed for the first excited state (the default if the Root option is not included), which is the one in which we are interested. 

This exercise would be a good place to compute a charge distribution difference density between the ground stale and the first excited state, and plot it using an available graphics package. 

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