Double resonance spectra calculation

Figure 12-3. IR-UV double resonance spectrum of GC (structure C) in the mid-IR frequency range (recorded at the FELIX free electron laser facility), compared with three types of ab intio calculations. Harmonic frequencies were obtained at the RI-MP2/cc-pVDZ, RI-MP2/TZVPP, and semiempirical PM3 levels of electronic structure theory. Anharmonic frequencies were obtained by the CC-VSCF method with improved PM3 potential surfaces [30]...

Many workers have in fact used density matrix methods for the calculation of line shapes and intensities in multiple resonance experiments, and two excellent reviews of the background theory are available. (49, 50) In addition there is also a simple guide (51) to the actual use of the method which is capable of predicting the results of quite elaborate experiments. Major applications have included the calculation of the complete double resonance spectrum from an AX spin system which gives 12 transitions in all (52) an extremely detailed study of the relaxation behaviour of the AX2 systems provided by 1,1,2-trichloroethane and 2,2-dichloroethanol (53) the effects of gating and of selective and non-selective pulses on AB and AX spin systems and the importance of the time evolution of the off-diagonal elements of the density matrix in repetitively pulsed FT NMR and spin-echo work (54) the use of double resonance to sort out relaxation mechanisms and transient responses (55) the calculation of general multiple resonance spectra (56) and triple resonance studies of relaxation in AB and AX spin systems. (57)... 

Copper porphyrin is one of the best-characterized of the metalloporphyrins, and its electron spin resonance (ESR) spectrum has been known for a quarter of a century.(17) More recently, electron nuclear double resonance (ENDOR) investigations have provided the complete hyperfine tensors for the metal, the nitrogens and the pyrrole protons.(18) We have used this detailed knowledge earlier(, ) to assess the quality of scattered-wave calculations. 

Figure 12-2 shows data we obtained for three isolated GC base pair structures. Row A shows results for the Watson-Crick (WC) structure, while rows B and C represent the second and third lowest energy structures, respectively, which are not WC. The second column shows the IR-UV double resonance data, compared with the ab initio calculations of the vibrational frequencies. These data allow us to assign the structures. The third column shows the UV excitation spectra, measured by resonant two-photon ionization (R2PI). The UV spectrum is broad for the WC structure (A) and exhibits sharp vibronic lines for the other structures. 

The microwave spectra and ab initio calculations (MP2/6-31G ) of 1-nitropyrazole indicate the planar structure of the molecule [1022], The initial assignment of the spectrum was carried out using a radio frequency-microwave double resonance technique. 

The complex proton magnetic resonance spectrum of 1,3-butadiene has been analyzed, and the calculated spectrum for this AA BB CC system has been determined (24,25). The temperatme-sensitive coupling constant for the protons on carbons 2 and 3 suggests an equilibrium between the predominant s-trans con-former and a skewed conformer having out-of-plane double bonds (25). The comparatively simple spectrum shows little effect of conjugation on the chemical shifts of the carbons (26). 

Fig. 4 IR-UV double resonance speclnun of adenine with the R2PI laser tuned to (a) 36,105 cm and (b) 35,824 cm. The stick spectrum shows the vibrational frequencies calculated at the B3LYP/ 6-311G(d,p) level. Data from PlUtzer and Kleinermanns (reprinted with permission, color coding added) [42]. Stick speara were computed at the B3LYP/6-31G(d) level with a scaling factor of 0.9613...

Some other molecular ions that have been studied more recently are listed in Table XII. Some examples of ionic complexes are also included in the table. In some cases, the analysis of the rotational spectrum was aided by the use of double resonance techniques or by information obtained from ab initio calculations. Most ions have been positive species (cation). Recently, the negative ion (anion) SH has been identified in the laboratory. The ion was produced by the low-pressure electric discharge of H2S and argon. The measurement of this and other light hydrides has been aided by the availability of higher frequency microwave sources. The first two transitions of SH fall at 0.56 and 1.1 THz (see Section XI). 

Simple resonance theory predicts that pentalene (48), azulene (49), and heptalene (50) should be aromatic, although no nonionic canonical form can have a double bond at the ring junction. Molecular orbital calculations show that azulene should be stable but not the other two, and this is borne out by experiment. Heptalene has been prepared but reacts readily with oxygen, acids, and bromine, is easily hydrogenated, and polymerizes on standing. Analysis of its NMR spectrum shows that it is... 

Fig. 2 (a) DRAMA pulse sequence (using % = t/2 = rr/4 in the text) and a representative calculated dipolar recoupled frequency domain spectrum (reproduced from [23] with permission), (b) RFDR pulse sequence inserted as mixing block in a 2D 13C-13C chemical shift correlation experiment, along with an experimental spectrum of 13C-labeled alanine (reproduced from [24] with permission), (c) Rotational resonance inversion sequence along with an n = 3 rotational resonance differential dephasing curve for 13C-labeled alanine (reproduced from [21] with permission), (d) Double-quantum HORROR experiment along with a 2D HORROR nutation spectrum of 13C2-2,3-L-alanine (reproduced from [26] with permission)... 

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