Rapid survey of nuclear quadrupole resonance by broadband excitation with comb modulation and dual-mode acquisition.

Nuclear Quadrupole Resonance (NQR) provides spectra carrying information as to the electric-field gradient around nuclei with a spin quantum number I > 1/2 and offers helpful clues toward characterizing the electronic structure of materials of chemical interest. A major challenge in NQR is finding hitherto unknown resonance frequencies, which can scatter over a wide range, requiring time consuming repetitive measurements with stepwise frequency increments. Here, we report on an efficient, two-step NQR protocol by bringing rapid-scan and frequency-comb together. In the first step, wideband excitation and simultaneous signal acquisition, both realized by a non-adiabatic, frequency-swept hyperbolic secant (HS) pulse with comb modulation, offers a clue for the existence/absence of the resonance within the frequency region under investigation. When and only when the sign of the resonance has been detected, the second step is implemented to compensate the limited detection bandwidth of the first and to unambiguously determine the NQR frequency. We also study the spin dynamics under the comb-modulated HS pulse by numerical simulations, and experimentally demonstrate the feasibility of the proposed scheme, which is referred to as RApid-Scan with GApped excitation with Dual-mode Operation (RASGADO) NQR.

Regularized regression analysis of digitized molecular structures in organic reactions for quantification of steric effects.

In organic chemistry, Comparative Molecular Field Analysis (CoMFA) can be defined as a regression analysis between reaction outcomes and molecular fields, wherein we can extract and visualize important structural information from the coefficients of the constructed regression models. In CoMFA, partial least-squares (PLS) regression, which determines all coefficients in the model, is used for fitting the regression models. However, in organic reactions, steric effects are observed only near the reactive site, indicating that a large number of regression coefficients in the CoMFA of organic reactions should be assigned as 0. The regularized regression method, LASSO/Elastic Net, allows us to fit the regression model while assigning 0 values to unimportant coefficients. Although LASSO/Elastic Net should be suitable for CoMFA, there is no example of its use for organic reaction analysis. Herein, we examine the performance of LASSO/Elastic Net for the quantification of steric effects in CoMFA. We employ digitized molecular structures (the indicator field) as molecular fields that represent steric effects. LASSO/Elastic Net regressions provide highly interpretable models that include less noise than those from PLS regression. © 2017 Wiley Periodicals, Inc.