Fundamental Intramolecular and Intermolecular Information from NMR in the Gas Phase

Abstract

Since theories on the behavior of dilute gases are in an advanced stage compared to condensed phases and complex materials, gas phase NMR measurements offer opportunities unique to the gas phase for fundamental understanding of the dependence of NMR quantities (shielding, J coupling) on the internal coordinates of the molecule (the property surfaces that mathematically describe how these molecular electronic properties change while the molecule carries out vibrational motions). The virial expansion is valid in the dilute gas, so that observations in the linear-density regime uniquely permit the determination of the linear-density dependence of NMR quantities, the second virial property coefficients that can be calculated in closed form by direct integration, if the intermolecular potential energy surface (PES) is known. Precise measurements afforded by NMR provide tested general theoretical treatments of intermolecular effects and intramolecular averaging that apply also to other molecular electronic properties. Spin–lattice relaxation times in the linear-density regime also provide a direct connection to the intermolecular PES; they can be calculated in terms of two well-defined cross-sections involving changes in molecular orientation and molecular rotational angular momentum sensitive to the anisotropy of the PES by well-established theory. Finally, observations of chemical exchange processes in gas phase NMR provide tests of kinetic theories for molecular rearrangements (RRKM) and transition state theory, and so provide tests of calculated transition states, reaction surfaces, and activation thermodynamic quantities such as ΔG⧧, ΔH⧧, ΔS⧧.

Solid-State NMR Investigations of Carbon Dioxide Gas in Metal–Organic Frameworks: Insights into Molecular Motion and Adsorptive Behavior

“…It is important to note that extensive work has been conducted by leaders in shale gas and xenon NMR research fields that can greatly benefit the interpretation of relaxometry data of CO 2 and other gases. We have found these references to be particularly helpful: Gas Phase NMR edited by Karol Jackowski and Micha Jaszuski (2016) contains many useful chapters; a basic introduction to models that describe the molecular reorientation of gases is found in ref ; in ref the investigators have conducted a number of experimental and theoretical studies and reviews that not are not specific to CO 2 ; and there is substantial shale gas literature published in the 20th century that establishes the basic behavior of CO 2 at various pressures and temperatures as a pure species and within rocks.…”

“…R 1 as a function of molecular loading or the adsorbed gaseous density trends as a hyperbola with a maximum, or T 1 trends as a hyperbola with a minimum. If an R 1 maximum is observed in a pure gas system, the maximum point is associated with the collision rate between molecules . In the low density regime of pure gases, the slope of R 1 versus ρ is associated with the rate of binary collisions.…”

Collisional cross-section of water molecules in vapour studied by means of 1H relaxation in NMR

“…Such relaxation rates have been measured over a wide range of pressures and temperatures8910. Experimentally determined cross-sections can be used to refine intermolecular potentials111213. In methane, isotopic substitution141516 affects relaxation rates associated with the different isotopomers such as CH 4 , CH 3 D, CH 2 D 2 , and CHD 3 .…”

Absolute 13C nuclear magnetic shielding of simple isolated molecules from gas phase measurements

“…where s means the appropriate shielding constants for nuclei with spin numbers I X,Y which are examined. The best choice to fulfil the above equation is to use frequencies extrapolated to the zero-pressure limit, measured as follows: 29 n( 13 C) = n 0 ( 13 C) + n 1 ( 13 C)r(X)…”

“…Comparing two resonance frequencies for two different nuclei present in gaseous mixture components, in the same uniform magnetic field, we can calculate the investigated dipole moment from the formula: 28 where σ means the appropriate shielding constants for nuclei with spin numbers I X , Y which are examined. The best choice to fulfil the above equation is to use frequencies extrapolated to the zero-pressure limit, measured as follows: 29 ν ( 13 C) = ν 0 ( 13 C) + ν 1 ( 13 C) ρ (X)…”