Temperature and Isotope Effects

Isotope effects have been used to provide information on the electronic transmission paths between the metal and the nuclei in first and second coordination spheres. Thus, the two-bond deuterium isotope effect A( H) on going from C5H5V(CO)4 to C5D5V(CO)4 (—0.75 ppm per has been discussed in terms of non-negligible overlap between hydrogen and metal orbitals [33]. The distinct two-bond coupling derived from relaxation 

For ionic complexes, or compounds carrying polar groups, hydrogen bonding to polar solvent molecules may occur and influence the shielding of the metal nucleus. This has been demonstrated for [V(CO)6]. This anion is subjected to a solvent isotope effect of —0.60 ppm per in D2O, which compares with —0.01 per in the non-polar toluene- fg [33]. A counter-ion effect of the same order of magnitude has also been noted. 

Tp = ti -C7H7, trimpsi = fBuSi(CH2PMc2)j. Cp = n CsMcj, p = oligodentate phosphine coordinating with m of the phosphorus functions dmpe = MejP(CHj)2PMcj, dppe = PhjP(CH2)2PPh2. 1/ if not indicated otherwise. 

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Thkusagawa F, Koetzle TF (1979) Neutron diffraction study of quinolinic acid recrystallized from D20 evaluation of temperature and isotope effects in the structure. Acta Cryst B35 2126-2135... 

Jou F.-Y., Freeman G.R., Temperature and isotope effects on the shape ofthe optical absorption spectrum of solvated electrons in water, J. Phys. Chem., 1979,83,2383-2387. 

Calculations of the spectra of the closely related compounds—acetylsalicylic acid (aspirin) and salicylaldehyde, at different temperatures and with selective deuteration, have been published for acetylsalicylic acid in Ref. 54 and for salicylaldehyde in Ref. 53. In the acetylsalicylic acid crystal there are no intramolecular hydrogen bonds—only intermolecular hydrogen bonds are present. The molecules of acetylsalicylic acid form hydrogen-bonded dimeric cyclic structures, similar to those present in salicylic acid. The IR spectra of the acetylsalicylic acid and the salicylic acid crystals in the region of the O—H(D) stretching vibrations are similar, excluding peaks due to intramolecular hydrogen bonds. Spectra of the acetylsalicylic acid crystal were calculated at two temperatures 300 and 77K. The experimental spectra and the observed temperature and isotopic effects are reproduced in these calculations reasonably well [54]. 

Here Tq are coordinates in a reference volume Vq and r = potential energy of Ar crystals has been computed [288] as well as lattice constants, thermal expansion coefficients, and isotope effects in other Lennard-Jones solids. In Fig. 4 we show the kinetic and potential energy of an Ar crystal in the canonical ensemble versus temperature for different values of P we note that in the classical hmit (P = 1) the low temperature specific heat does not decrease to zero however, with increasing P values the quantum limit is approached. In Fig. 5 the isotope effect on the lattice constant (at / = 0) in a Lennard-Jones system with parameters suitable for Ne atoms is presented, and a comparison with experimental data is made. Please note that in a classical system no isotope effect can be observed, x "" and the deviations between simulations and experiments are mainly caused by non-optimized potential parameters. 

The hydrogen molecule does not chemisorb onto clean sintered gold surfaces at or above 78 K [147] but on unsintered films, a small amount of H2 is chemisorbed if gold surface atoms of low coordination number are present [148]. Stobinski [149] found that H2 can also chemisorb on thin sintered Au films if the surface is covered at low temperatures with a small amount of gold equivalent to 1-3 Au monolayers prior to H2 exposure. This suggests a fundamental role of surface Au atoms of low coordination number in the chemisorption process. Deuterium molecules also chemisorb in a similar fashion on gold films at 78 K and isotope effects were... 

Kinetic complexity can produce apparently temperature-independent isotope effects. For example, a rise in temperature produces a smaller intrinsic isotope effect, in agreement with the conventional expectations of Chart 3, for an isotope-sensitive step that is partially rate limiting. If at the same time the rise in temperature makes other steps relatively more rapid so that the isotope-sensitive step then becomes more nearly rate-limiting, then the intrinsic isotope effect will be more fully expressed (Chart 4). If these effects roughly balance, then the isotope effect may appear to be independent of temperature while in fact fully in accord with semiclassical expectations. Seymour and Klinman have discussed in detail the problem of kinetic complexity in isotope-effect temperature dependences. 

Because both the passive fluctuations and the modulating vibrations can require thermal excitation, this model is capable of accounting for temperature-dependent isotope effects, including those traditionally described by the BeU model. Theoretical studies, which will be the topic of the second and third parts of this three-part series of articles, have not yet produced a consensus on the contribution of specific protein motions to enzyme catalysis. 

Knapp, M.J., Rickert, K. and Khnman, J.P. (2002). Temperature-dependent isotope effects in soybean lipoxygenase-1 correlating hydrogen tunneling with protein dynamics. J. Am. Chem. Soc. 124, 3865-3874... 

Romanek C. S., Grossman E. L. and Morse J. W. (1989). Carbon isotope fractionation in aragonite and calcite Experimental study of temperature and kinetic effects. Geol Soc. Amer. Abstr. Prog., 2LA76. 

It is expected that the temperature of deep-water masses is more or less constant, as long as ice caps exist at the poles. Thus, the oxygen isotope composition of benthic organisms should preferentially reflect the change in the isotopic composition of the water (ice-volume effect), while the 5 0-values of planktonic foraminifera are affected by both temperature and isotopic water composition. 

Isotope effects have also been investigated for the case of hydrogen-deuterium substitution in (C6H5)2PH, C6H5PH2,(1967 30) HPO20, HP(0)(0CH3)2 and other examples. a967 42) Lastly, temperature and solvent effects have also been considered for a variety of cases. (1967>4310... 

Orientation in the HD Addition to Methyl Acetylene and Allene, and Isotope, Effects in the Addition of //2 and D2 at Room Temperature on MoS2... 

Figure 20. The IR Vx-h line shapes of gaseous cyclic acetic acid dimers and isotope effect at room temperature.

Figure 24. Effects of temperature and isotopic substitution on the spectral densities of crystalline adipic acid in the absence of Fermi resonance. Comparison between theoiy (Eq. (279)) (thick Line) and experiment [101] (grayed spectra). Left column calculations using the breaking of the IR selection rule (r)° = 0). Right column same calculations but without the breaking of the IR selection rule (r 0 = 0).

The first step in quadrupole resonance studies is the detection of lines in new compounds and the interpretation of the observed frequencies in the light of the molecular and crystalline structure of the compound. In the second step, physical studies are developed which yield informations on the temperature and pressure effects, on the isotopic shift, and on the Zeeman and Stark effects. 

Niobium-93 NMR data other than chemical shifts are even more scarce. The information on temperature gradients tg, coupling constants J and isotope effects A available to date is sununarized in Table 4. The temperature behaviour of Nb shielding and linewidth compares with that discussed for in Section 2.1.3. The trinuclear complex Cp3Nb3(CO)7 (20), with one of the... 

In addition to concentration there are essentially four reaction variables that can be relatively easily controlled and that may have a considerable effect on the course of a photochemical reaction these are the reaction medium and temperature, and the wavelength and intensity of the exciting light. In addition, magnetic Held and isotope effects may come into play. 

Although the nuclear partition function is a product of the translational, rotational, and vibrational partition functions, the isotope effect is determined almost entirely by the latter, specifically by vibrational modes involving motion of isotopically different atoms. In the case of light atoms (i.e., protium vs. deuterium or tritium) at moderate temperatures, the isotope effect is dominated by ZERO-POINT energy differences. 

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