Polyolefins are important and broadly used materials. Their molecular microstructures have direct impact on macroscopic properties and dictate end-use applications. 13 C NMR is a powerful analytical technique used to characterize polyolefin microstructures, such as long-chain branching (LCB), but it suffers from low sensitivity. Although the 13 C sensitivity of polyolefin samples can be increased by about 5.5 times with a cryoprobe, when compared with a conventional broadband observe (BBO) probe, further sensitivity enhancement is in high demand for studying increasingly complex polyolefin microstructures. Toward this goal, distortionless enhancement by polarization transfer (DEPT) and refocused insensitive nuclei enhanced by polarization transfer (RINEPT) are explored. The use of hard, regular, and new short adiabatic 180°1 3 C pulses in DEPT and RINEPT is investigated. It is found that RINEPTs perform better than DEPTs and a sensitivity enhancement of 3.1 can be achieved with RINEPTs. The results of RINEPTs are further analyzed with statistics software JMP and recommendations for optimal usage of RINEPTs are suggested. An example of analyzing saturated chain ends in an ethylene−octene copolymer sample with a hard 180°1 3 C RINEPT pulse is demonstrated. It is shown that the experimental time can be further reduced in half because of faster proton relaxation, where the total experimental time is about 580 times shorter when compared to using a conventional method and a 10 mm BBO probe. A naturally abundant nitrogen-containing polyolefin is analyzed using 1 H− 15 N HMBC and, to our knowledge, is the first 1 H− 15 N HMBC presented in the field of polyolefin characterization. The relative amount of similar nitrogen-containing structures is quantified by two-dimensional integration of 1 H− 15 N HMBC. Two pragmatic technical challenges related to using high-sensitivity NMR cryoprobes are also addressed: (1) A new 1 H decoupling sequence Bi_Waltz_65_256pl is proposed to address decoupling artifacts in 13 C{ 1 H} NMR spectra which contain a strong 13 C signal with a high signal-to-noise ratio (S/N). (2) A simple pulse sequence that affords zero-slope spectral baselines and quantitative results is presented to address acoustic ringing that is often associated with high-sensitivity cryoprobe use.

The electronic structures of phenylnitrenes with anionic π-donating substituents are investigated by using mass spectrometry and electronic structure calculations. Reactions of para-CH(2)(-)-substituted phenylnitrene, formed by dissociative deprotonation of p-azidotoluene, with CS(2) and NO indicate that it has a closed-shell singlet ground state, whereas reactions of p-oxidophenylnitrene formed by dissociative deprotonation of p-azidophenol indicate either a triplet ground state or a singlet with a small singlet-triplet splitting. The ground electronic state assignments based on ion reactivity are consistent with electronic structure calculations. The stability of the closed-shell singlet states in nitrenes is shown by Natural Resonance Theory to be very sensitive to the amount of deprotonated-imine character in the wave function, such that large changes in state energies can be achieved by small modifications of the electronic structure.

We describe the use of MALDI-TOF mass spectrometry and collision-induced dissociation (CID) fragmentation techniques to examine the thermally induced cross-linking chemistry of dibenzocyclobutene (BCB) 2 − resorcinol-based materials. The overall goal was to gain a better understanding of benzocyclobutene ring-opening and the subsequent Diels−Alder cycloaddition reactions, which could assist in troubleshooting problems associated with unwanted side-product formation in these materials. Experimental results indicate that relative changes in average molecular mass (e.g., M n ) and signature side-product formation can be used to predict the optimum curing temperature used for cross-linking/polymerization. CID fragmentation studies identified two low kinetic energy degradation pathways for the BCB 2 −resorcinol-based oligomers examined in this study: (1) ether bond cleavage with an associated 1,3-hydrogen transfer and (2) transannular bond cleavage across the cyclooctadiene linkage. This information was used to develop a general fragmentation model for BCB 2 −resorcinol-based materials to predict their degradation products and verify their chemical connectivity.

The absolute enthalpies of formation of 3,4-, 2,3-, and/or 2,4-didehydropyridines (3,4-, 2,3- and 2,4-pyridynes) have been determined by using energy-resolved collision-induced dissociation of deprotonated 2- and 3-chloropyridines. Bracketing experiments find the gas-phase acidities of 2- and 3-chloropyridines to be 383 ± 2 and 378 ± 2 kcal/mol, respectively. Whereas deprotonation of 3-chloropyridine leads to formation of a single ion isomer, deprotonation of the 2-chloro isomer results in a nearly 60:40 mixture of regioisomers. The enthalpy of formation of 3,4-pyridyne is measured to be 121 ± 3 kcal/mol by using the chloride dissociation energy for deprotonated 3-chloropyridine. The structure of the product formed upon dissociation of the ion from 2-chloropyridine cannot be unequivocally assigned because of the isomeric mixture of reactant ions and the fact that the potential neutral products (2,3-pyridyne and 2,4-pyridyne) are predicted by high level spin-flip coupled-cluster calculations to be nearly the same in energy. Consequently, the enthalpies of formation for both neutral products are assigned to be 130 ± 3 kcal/mol. Comparison of the enthalpies of dehydrogenation of benzene and pyridine indicates that the nitrogen in the pyridine ring does not have any effect on the stability of the aryne triple bond in 3,4-pyridyne, destabilizes the aryne triple bond in 2,3-pyridyne, and stabilizes the 1,3-interaction in 2,4-pyridyne compared to that in m-benzyne. Natural bond order calculations show that the effects on the 2,3- and 2,4-pyridynes result from polarization of the electrons caused by interaction with the lone pair. The polarization in 2,4-pyridyne is stabilizing because it creates a 1,2-interaction between the nitrogen and dehydrocarbons that is stronger than the 1,3-interaction between the dehydrocarbons.