Formation of intermolecular hydrogen bonds in light-sensitive crystals of C-2′-(o-oxyphenyl)vinyl-N-p-methylphenyl nitrone, C-2′-(2″-oxy-5″-bromophenyl)vinyl-N-p-methylphenyl nitrone, C-2′-(2″-oxy-5″-bromophenyl)-vinyl-N-phenyl nitrone, and C-2′-(o-oxyphenyl)vinyl-n-methyl nitrone