NBO, HOMO–LUMO, conformational and vibrational spectroscopic study of 2-(3,4-dihydroxyphenyl)-5,7-dihydroxy-3-[(2S,3R,4S,5R,6R)-3,4,5-trihydroxy-6-(hydroxymethyl) oxan-2-yl]oxychromen-4-one, as the main phenolic component of the extracts of walnut leaves by DFT