Close Relationships between NMR J-Coupling Alternation (JCA) and Molecular Properties of Carbon Chains

Abstract

We propose a J-coupling alternation (JCA) value that is demonstrated to be a suitable parameter to evaluate the nuclear magnetic resonance (NMR) indirect spin−spin coupling constants (SSCCs) as a function of molecular properties of chains by increasing their length. As an application, we report a theoretical study of the SSCCs for the interactions between neighbor nuclei in increasingly patterned carbon chains within density functional theory. First, we examine the J-coupling constants between 1 H and 13 C nuclei ( n J HC ) considering the separation distance, as well as between two adjacent 13 C nuclei ( 1 J CC ) considering their relative positions in polyynes and cumulenes. Further, we define and determine JCA in terms of the differences of 1 J CC , which is investigated as a function of several molecular properties, e.g., cohesive energy, characteristic vibrational frequency, average polarizability, and energy gap of the systems. We also determine JCA for other types of carbon chains, such as diphenyl-capped polyynes, polyacetylene and polythiophene. The behavior of JCA as a function of the energy gap may be related to highly π-conjugated low-band-gap carbon chains. Overall, JCA correlates very well with the electronic properties of these chains, especially with their energy gap, exhibiting positive values for pristine polyyne and polythiophene and negative values for pristine cumulene and plyacetylene. These findings indicate an alternative way to establish an appropriate SSCC descriptor that characterizes the electronic nature of the system, such as the proposed JCA value averaging the whole system, instead of using only the individual J-coupling values to give insights into the properties of large and extended systems.

Linear response of the indirect J‐coupling alternation to the energy gap of increasing π‐conjugated oligomers

“…As a compromise between the NMR computational calculations and the model accuracy, we have employed the 6‐311G(d,p) basis set to expand the Kohn‐Sham orbitals for all electrons. In our previous work, [ 29 ] tests for small oligomers using adapted basis sets for SSCC calculations [ 43 ] indicate that the present results of the J ‐coupling constants are sufficiently accurate for this purpose. We have calculated all SSCCs within this scheme and evaluated the oscillatory behavior of the J ‐coupling constants, between two neighbor 13 C nuclei, considering their relative positions in the chains for all these oligomers (Figure 2).…”

“…Thus, is a single‐valued NMR‐based descriptor that can be related to the electronic properties of increasing π‐delocalized chains. [ 29 ] Notice that this parameter sums algebraically over all the 1 J CC coupling values of the system, including the terminal nuclei, thus giving an appropriate average value for the through‐bond interactions. [ 44 ] We show here that the JCA is a gap‐dependent variable or parameter, which correlates to other molecular properties, when calculated by increasing the length of the conjugated oligomers.…”

“…In this paper, we discuss the dependence of a novel parameter, [ 29 ] theoretically derived from the nuclear magnetic resonance (NMR) spectroscopy, on the calculated HOMO‐LUMO gap of increasing carbon chains of trans ‐PA, PPP, PPy, and PTh. This parameter is based on the alternation of the indirect spin‐spin coupling constants (SSCCs), characteristic of these types of chains.…”

“…This parameter is based on the alternation of the indirect spin‐spin coupling constants (SSCCs), characteristic of these types of chains. [ 29,30 ] Although the direct magnetic dipole‐dipole couplings have long been employed to determine the BLA, [ 31 ] here, we study the use of indirect J ‐coupling constants between two adjacent 13 C nuclei ( 1 J CC ) in conjugated oligomers. Thus, we define a scalar J ‐coupling alternation (JCA) parameter [ 29 ] in terms of the differences of 1 J CC , which is investigated as a function of the electronic energy gaps, as well as other properties of the oligomers.…”

“…[ 29,30 ] Although the direct magnetic dipole‐dipole couplings have long been employed to determine the BLA, [ 31 ] here, we study the use of indirect J ‐coupling constants between two adjacent 13 C nuclei ( 1 J CC ) in conjugated oligomers. Thus, we define a scalar J ‐coupling alternation (JCA) parameter [ 29 ] in terms of the differences of 1 J CC , which is investigated as a function of the electronic energy gaps, as well as other properties of the oligomers. We show that, for all these systems ( trans ‐PA, PPP, PPy, and PTh), the HOMO‐LUMO gaps may exhibit a fairly linear response on the JCA, indicating that this parameter properly depicts the electronic features of the chemical bonds in highly conjugated systems.…”