Molecular dynamics fluxionality

13C shifts respond to pH changes with even greater sensitivity this is demonstrated by the values of pyridine (39b) and its cation (39a). 

The effect of pH is rarely of use for pK measurement it is more often of use in identifying the site of protonation/deprotonation when several basic or acidic sites are present. Knowing the incremental substitutent effects Z 4 6 of amino and ammonium groups on benzene ring shifts in aniline and in the anilinium ion (40), one can decide which of the nitrogen atoms is protonated in proca-ine hydrochloride (problem 24).---------------------------------------------------------------- 

The dimethylamino group rotation follows a first-order rate law the exchanging methyl protons show no coupling and their singlet signals are of the same intensity. Under these conditions, equation 7 2 35-37 affords the rate constant kr at the coalescence point Tc  

Hence the activation energy barrier to dimethylamino group rotation in dimethylacetamide (41) is calculated from equation 9 with kr = 17.8 s 1 at the coalescence point 353 K (Fig, 2.26)  

Temperature-dependent (dynamic) NMR studies are suited to the study of processes with rate constants between 10 and 103 s 1 3. Some applications are shown in Table 2.13 and in problems 13 and 14. 

Rotation hindered by bulky substituents (f-butyl groups) 

---

The CH cation 1, protonated methane, is the parent of hypercoordinated carbocations containing a five coordinated carbon atom. It is elusive in solution and has not been observed by NMR spectroscopy but gas-phase infrared investigations have shown its fluxional structure which has been proven by ab initio molecular dynamic simulation.18... 

Numerous studies concerning the structure and dynamics of the CH5+ have been reported (5-7, 16, 18, 67-74). Figure 2 shows the Bom-Oppenheimer molecular dynamics (BOMD) calculations for CHS+, calculated at B3LYP/6-31G level. The structures (a) and (b) correspond to snapshots from the dynamics. Figure 2 (c) shows the superimposed structures from the dynamics, indicating the high fluxionality of CH5+. 

Molecular Dynamics of H FeR CCO)-] and Related Mixed-Metal Clusters. Metal clusters have been shown to undergo a wide variety of fluxional processes in which carbonyls, hydrides, and even the metals themselves undergo rearrangement (25). Mixed-metal clusters are ideally suited for studies of fluxional processes because of the low symmetry which is inherent within their metal framework. In such clusters, the majority of... 

Since the aim of the dynamic investigation is the quantification of the time scale of the motion and the geometric interpretation of the molecular process, the type and the correlation time of dynamics determine the choice of experiments and techniques. The SSNMR represents a powerful tool for the investigation of molecular dynamics due to the possibility offered by the use of different parameters to cover a wide timescale of a fluxional process from 10 ... 

Mann has applied the Forsen-Hoffman spin saturation method using C n.m.r. spectroscopy to the solution of several problems of molecular dynamics ie.g. see ref. 1). Using this technique, the cyclo-octatetraene complex [Cr(CO)3(tj -cot)] is shown to be fluxional by means of 1,3- and, to a lesser extent, 1,2-shifts. This new evidence clearly disproves the suggestion that random shifts are occurring in this complex but supports Whitesides argument for 1,3-shifts in which a 16-electron intermediate [Cr(CO)3(rj -cot)] may participate (see Vol. 5). In fact careful analysis of variable-temperature n.m.r. lineshapes does support a non-random fluxional process and at 26.5 °C gives rate constants for 1,3- and 1,2-shifts Ati,3=48 s and A i, = 24 s-i. ... 

---