Abstract
Two new aromatic pyrimidine-based derivatives designed specifically for halogen bond directed self-assembly are investigated through a combination of high-resolution Raman spectroscopy, X-ray crystallography, and computational quantum chemistry. The vibrational frequencies of these new molecular building blocks, pyrimidine capped with furan (PrmF) and thiophene (PrmT), are compared to those previously assigned for pyrimidine (Prm). The modifications affect only a select few of the normal modes of Prm, most noticeably its signature ring breathing mode, ν1. Structural analyses afforded by X-ray crystallography, and computed interaction energies from density functional theory computations indicate that, although weak hydrogen bonding (C–H···O or C–H···N interactions) is present in these pyrimidine-based solid-state co-crystals, halogen bonding and π-stacking interactions play more dominant roles in driving their molecular-assembly.
Highlights
It is well-accepted in the field of crystal engineering that noncovalent interactions play dominant roles in directing self-assembly [1,2,3,4,5,6]
We previously characterized the effects of noncovalent interactions on ten normal modes of pyrimidine in the region [42,43,44,45], and Figure 3 compares the Raman spectra in regions of these modes in greater detail
Exhibit additional peaks because of the motions involving furan and thiophene, and the peaks originating from the frequencies that have shifted upon the addition of the furan and thiophene groups
Summary
It is well-accepted in the field of crystal engineering that noncovalent interactions play dominant roles in directing self-assembly [1,2,3,4,5,6]. A halogen bond is a noncovalent interaction involving the net attraction between an electrophilic region of a halogen atom (e.g., bromide or iodide; C–Br or C–I) on one molecule, and the nucleophilic region of another or the same molecule [20]. Electron withdrawing atoms such as fluorine or similar functional groups on the halogen bond donor have been shown to significantly enhance this σ-hole interaction, to the point of even energetically competing with hydrogen bonding interactions [21]. Pyridine- and pyrimidine-based derivatives make ideal halogen bond accepting building blocks, and a number of architectures incorporating various combinations of donors and acceptors have been introduced in recent years [17,18,19]
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