Geometry and Raman spectra of P.R. 255 and its furo-furanone analogue

Structure and Raman spectra of pyridyl substituted diketo-pyrrolo-pyrrole isomers and polymorphs

“…There is only one observed peak corresponding to C@O stretching for all compounds except 4PyPP4Py (phase II). The experimental values for 4PyPPB (1667 cm À1 ) and 4PyPP4Py (phase I, 1666 cm À1 ) are almost the same as for BPPB (1666 cm À1 ) [9], reflecting thus strong in-plane COANH H-bonding in both crystals. Corresponding values for 2PyPPB (1686 cm À1 ) and 2PyPP2Py (1704 cm À1 ) are considerably higher, reflecting weaker out-of-plane or completely missing COANH hydrogen bonding, respectively.…”

“…No such mutually excluding rule exist for C s group; both planar a 0 modes and out-of-plane a 00 modes can be Raman active. As compared to BPPB [9], two (one) CAH stretching, CAH in-plane bending and CAH out-of-plane bending modes are missing in theoretical calculations of (di)pyridyl derivatives. Other modes of pyridyl substituted DPPs have their counterparts in theoretical spectra of BPPB.…”

“…Their agreement with the experimental ones thus depends among others on a similarity of computed and real geometry. DFT method with the most widely used B3LYP xc functional [18] and its combination with 6-311++G(d,p) basis set was successfully used in our previous studies [9,10]. Calculations for 4-pyridyl derivatives were straightforward and resulted in a strictly planar geometry for 4PyPP4Py and thus C 2h symmetry.…”

“…4 including the symmetry constraints. We do not use scaling of theoretical frequencies for the reasons explained elsewhere [9].…”

“…While NH(pyrrolinone) protons take part exclusively in COANH hydrogen bonds in phase I, half of them is H-bonded to N(pyridine) in phase II. We have recently shown [9] on BPPB, that the comparison of the experimental Raman spectra with the theoretical ones obtained by quantum chemical calculations is a valuable tool for an investigation of intermolecular hydrogen bonding between CO and NH functional groups. The same methodology was successfully applied to isoindigo [10].…”