Question

Can someone explain in simple way. I dont know how much bromobenzene and CDCl3 needed in NMR who know physcial chemistry and what to do next. and expreiment name is

Anisotropic Rotational Diffusion Studied by Nuclear Spin Relaxation and Molecular Dynamics Simulation: An Undergraduate Physical Chemistry Laboratory

NMR T1 Experiments

Samples were prepared in advance and consisted of 50% by volume bromobenzene in CDCl3 sealed in 5 mm NMR tubes after degassing with three freeze−pump−thaw cycles. Proton decoupled 13C inversion recovery experiments were performed on a Bruker Avance 400 spectrometer using TopSpin 2.1 software. Twelve delay times were used with a maximum value of 75 s (approximately 5T1). Collecting eight transients per spectrum led to S/N of approximately 1000 for the fully relaxed spectra and required 2.5 h for the complete experiment. Rather than using TopSpin’s built-in T1 calculation, peak amplitudes were read manually and the data were fit in Mathematica20 using NonLinearModelFit. This is a pedagogical choice designed to make the students interact more closely with the data and to allow control over the type of error estimates obtained. D∥ and D⊥ were calculated from the T1 values of the meta and para carbons.

MD Simulations

Simulations were run using Gromacs 5.1 running on Linux based personal computers. Structure files of bromobenzene and an equilibrated CDCl3 solvent box were prepared in advance. Students construct a 4.0 nm cube containing the appropriate mole ratio of solute and solvent (e.g., 183 bromobenzene and 240 CDCl3). After several equilibration steps, a 200 ps trajectory was calculated using the OPLS-AA force field11 with a 1 fs step size and NVT conditions (constant number of molecules, volume, and temperature). Lengths of equilibration steps and the final trajectory were chosen to minimize computation time while not affecting the final correlation times obtained by more than 5%. NVT conditions were chosen to most nearly replicate the environment of a sealed NMR tube under thermostatic control. (Strictly speaking, linear momen- tum is conserved only in simulations run under NVE conditions, and so simulations probing dynamics would be best run under these conditions. In simulations run under NVT or NPT conditions, linear momentum is not conserved but is instead coupled to “bath variables”, which reasonably leads to concerns about whether the dynamical properties of the system are effected. In practice, however, the selection of different ensembles often leads to identical results.) Atomic coordinates were stored every 100 fs. Correlation functions were calculated from the atomic coordinates of the trajectories using a Fortran program provided by the instructor. Correlation times can be obtained by direct numerical integration of the correlation function. The diffusion rates are then calculated from the correlation times using expressions given in eq 5. All input files required for the MD simulation are provided in the Supporting ■Informatio