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Isotropic nuclear shielding constants at the equilibrium molecular structure σeq and zero-point vibrational corrections (ZPVCs) to σeq are evaluated using the B3LYP/aug-cc-pVTZ level of theory, as well as the KT2/aug-cc-pVTZ level of theory. Various scaling factors and systematic corrections are obtained by linear regression to experimental shielding constants. Comparisons of the scaled and systematically corrected equilibrium and vibrationally averaged shielding constants reveal that, at the 99% confidence level, the ZPVCs via second-order perturbation theory do not improve the agreement of B3LYP/aug-cc-pVTZ and KT2/aug-cc-pVTZ calculated shielding constants with experiment. This holds true when the same analysis is applied to CCSD(T)/aug-cc-pCV[TQ]Z calculated σeq of Teale et al. [ Journal of Chemical Physics 2013, 138, 024111]. In addition, at the 99% confidence level, B3LYP/aug-cc-pVTZ and KT2/aug-cc-pVTZ scaled and systematically corrected shielding constants are found to be statistically no different from CCSD(T)/aug-cc-pCV[TQ]Z calculated σeq. The use of scaling factors and systematic corrections could thus provide a cheaper but yet reasonably accurate alternative for the study of nuclear shielding constants of larger systems. [ABSTRACT FROM AUTHOR]

Using the method of modified Shepard's interpolation to construct potential energy surfaces of the H2O, O3, and HCOOH molecules, we compute vibrationally averaged isotropic nuclear shielding constants σ(c) of the three molecules via quantum diffusion Monte Carlo (QDMC). The QDMC results are compared to that of second-order perturbation theory (PT), to see if second-order PT is adequate for obtaining accurate values of nuclear shielding constants of molecules with large amplitude motions. σ(c) computed by the two approaches differ for the hydrogens and carbonyl oxygen of HCOOH, suggesting that for certain molecules such as HCOOH where big displacements away from equilibrium happen (internal OH rotation), σ(c) of experimental quality may only be obtainable with the use of more sophisticated and accurate methods, such as quantum diffusion Monte Carlo. The approach of modified Shepard's interpolation is also extended to construct shielding constants σ surfaces of the three molecules. By using a σ surface with the equilibrium geometry as a single data point to compute isotropic nuclear shielding constants for each descendant in the QDMC ensemble representing the ground state wave function, we reproduce the results obtained through ab initio computed σ to within statistical noise. Development of such an approach could thereby alleviate the need for any future costly ab initio σ calculations. [ABSTRACT FROM AUTHOR]

Anthropogenic nutrient pollution has been identified as one of the key stressors of coastal ecosystems. However, the paucity of long-term nutrient records limits our understanding of both the extent of nutrient pollution as well as of the ecological impacts. Here, using coral skeletal phosphorus (P/Ca), we reconstructed a half-a-century record of seawater phosphate at Port Dickson, Malaysia. The P/Ca in the coral revealed an up to 8-fold increase in coral P/Ca from the late 1970s to 2000s, likely linked to increases in fertilizer use (R2 = 0.47) and variabilities in rainfall (R2 = 0.17). The rise in coral P/Ca in coincided with a contemporaneous 18 % decrease in coral skeletal density, suggesting phosphate enrichment may impact the growth and structural integrity of reef-building corals. Given the importance of both agriculture and heavy reliance on coral reefs by populations in Southeast Asia, our study highlights continue the need to develop environmental management upstream of coastal zones. [Display omitted] • A half-century record of coral P/Ca is reconstructed from Port Dickson, Malaysia. • Pre-pollution seawater phosphate baseline of ~0.1 μM as estimated from coral P/Ca. • The coral P/Ca increased up to 8-fold over the late 20th century. • The increase in P/Ca likely caused by land-based human activities and rainfall. • The rise in coral P/Ca coincided with 18 % decrease in coral skeletal density. [ABSTRACT FROM AUTHOR]

[Display omitted] • Salt formations enhance the stability of energetics materials. • The experimental impact sensitivity trend can qualitatively predict. • HOMO-LUMO energy gap, BDE/E, V m i d can use to predict the impact sensitivity. • Bond topology parameters can also use to predict the impact sensitivity. The higher impact sensitivity of many energetic materials has become one of the major obstacles which diminishes its wide range of applications in various fields. Salt formation is an effective approach that enhances the stability of the molecules towards impact. In this study quantum mechanical calculations have been employed in order to predict the impact sensitivity trends for the series of nitrogen rich energetic salts: 3-Amino-1,2,4(4H)-oxadiazol-5-one (AOD) and 4-Nitramino-1,2,4-Triazole (NRTZ). Quantum mechanically derived criteria, namely, HOMO-LUMO energy gap, ratio of the bond dissociation energy to molecular total energy, the electrostatic potential at bond mid-point, bond topological parameters were computed, and the quality of results are assessed against experimental BAM fall hammer test results. Several sets of DFT functionals and basis sets were tested to identify the best level of theory for the selected nitrogenous salts. The results demonstrated an excellent qualitative prediction of the impact sensitivity by using CAMB3LYP/6-31G(d)/IEFPCM = water level of theory. Hence, quantum mechanical predictions are an ideal preliminary approach to design advanced energetic salts based on AOD and NRTZ with enhanced stability prior to the synthesis, which facilitates reducing the great cost and risk to safety. [ABSTRACT FROM AUTHOR]