6-Methyl-2-butylureidopyrimidone dimerizes via four hydrogen bonds in the solid state as well as in CHCl3 solution via a donor−donor−acceptor−acceptor (DDAA) array of hydrogen bonding sites in the 4[1H]-pyrimidinone tautomer. An intramolecular hydrogen bond from the pyrimidine NH group to the urea oxygen atom preorganizes the molecules for dimerization. The dimerization constant of the dimer in CHCl3 exceeds 106 M-1. In CHCl3 containing DMSO, the dimer is in equilibrium with the monomeric 6[1H]-pyrimidinone tautomer. In 6-phenyl-2-butylureidopyrimidone, the 4[1H]-pyrimidinone tautomer coexists with the pyrimidin-4-ol form, which dimerizes with similar high dimerization constants via four hydrogen bonds in a DADA array. The latter tautomer predominates in derivatives with electronegative 6-substituents, like 6-nitrophenyl- and 6-trifluoromethyl-2-butylureidopyrimidone. Due to its simple preparation and high dimerization constant, the ureidopyrimidone functionality is a useful building block for supramolecu...

Strong dimerization of ureidopyrimidones via quadruple hydrogen bonding

3.3. Influence of the Bridge Redox States 5159 4. Multidimensional Organic Mixed Valence 5163 5. Intervalence Charge Transfer across Noncovalent Pathways 5167 6. Use of EPR Spectroscopy as a Direct Test for Hush Theory 5170 7. Solvent and Ion Pairing Effects 5171 8. Crystallographic Studies 5173 9. Concluding Remarks 5175 Author Information 5175 Biographies 5175 Acknowledgment 5176 References 5176

Organic mixed valence.

The base-stacking patterns in over 70 published crystal structures of nucleic acid constituents and polynucleotides were examined. Several recurring stacking patterns were found. Base stacking in the solid state apparently is very specific, with particular modes of interaction persisting in various crystalline environments. The vertical stacking of purities and pyrimidines in polynucleotides is similar to that observed in crystals of nucleic acid constituents. Only partial base overlap was found in the majority of the structures examined. Usually, the base overlap is accomplished by positioning polar substituents over the ring system of an adjacent base. The stacking interactions are similar to those found in the crystal structures of other polar aromatic compounds, but are considerably different from the ring–ring interactions in nonpolar aromatic compounds. Apparently, dipole-induced dipole forces are largely responsible for solid-state base stacking. It is found that halogen substituents affect base-stacking patterns. In general, the presence of a halogen substituent results in a stacking pattern which permits intimate contact between the halogen atom and adjacent purine or pyrimidine rings. Considering differences in the stacking patterns found for halogenated and nonhalogenated pyrimidines, a model is proposed to account for the mutagenic effects of halogenated pyrimidines.

Stereochemistry of Nucleic Acids and Their Constituents. X. * Solid-state Base-Stacking Patterns in Nucleic Acid Constituents and Polynucleotides

The intramolecular charge transfer (ICT) reaction from the locally excited state (LE) to the charge transfer state (CT) in the singlet excited state is investigated for the dual fluorescent 4-(dial...

Intramolecular charge transfer in dual fluorescent 4-(dialkylamino)benzonitriles.

The planar rigidized molecule fluorazene (FPP) undergoes fast reversible intramolecular charge transfer (ICT) in the excited state, with a reaction time of 12 ps in the polar solvent ethyl cyanide at −45 °C. The ICT state of FPP has a dipole moment μe(ICT) of 13 D, much larger than that of the locally excited state LE (1 D). The ICT behavior of FPP is similar to that of its flexible counterpart N-phenylpyrrole (PP), for which μe(ICT) = 12 D. These results show that intramolecular charge transfer to a planar ICT state can occur efficiently. In designing ICT systems capable of rapid switching, it is therefore important to realize that large amplitude motions such as those necessary for the formation of a twisted intramolecular charge transfer (TICT) state are not required.

Fast Intramolecular Charge Transfer with a Planar Rigidized Electron Donor/Acceptor Molecule

Diffractometer software (cad4f)

The crystal structure of the room- and low-temperature modifications of Würster's blue perchlorate, TMPD.ClO4. I. The room-temperature phase

(SnS)1.17NbS2, Mr = 334.92, is a compound with misfit layer structure consisting of two-atom-thick layers of SnS and three-atom-thick sandwiches of NbS2 which alternate along the c axis. The lattices of SnS and NbS2 both have C-centered orthorhombic unit cells which match along the b and c axes but not along the a axes. The unit cells and space groups are a = 5.673 (1), b = 5.750 (1), c = 11.760 (1) A, space group C2mb (no. 39), Z = 4, for SnS; a = 3.321 (1), b = 5.752 (1), c = 11.763 (1) A, space group Cm2m (no. 38), Z = 2, for NbS2. From the ratio of the lengths of the a axes of the two parts of the complete structure (5.673/3.321 = 1.708 being irrational) one obtains a composition (SnS)1.17NbS2. The structure determination consisted of three parts: the structures of the SnS and NbS2 parts separately and their relative position. Intensities were measured with Mo K{\bar \alpha} radiation (λ = 0.71073 A) at T = 293 K, μ = 102.3 cm-1. For the SnS part RF = 0.088 for 306 unique reflections; for the NbS2 part RF = 0.031 for 329 unique reflections (for both cases 0kl reflections excluded). The relative position of these two lattices was determined from the common reflections 0kl: RF = 0.072 for 98 reflections. The SnS part of the structure consists of deformed slices of SnS with a thickness of half the cell edge of (hypothetical) NaC1-type SnS. Each Sn atom is in this way coordinated by five sulfur atoms; four sulfur atoms are in a plane perpendicular to the c axis with SnS distances 2.74 (1) (1×), 2.913 (1) (2×) and 3.01 (1) A (1×), whereas the fifth Sn-S bond with length 2.695 (9) A is approximately along the c axis. The NbS2 part of the structure is that of NbS2-2H with Nb in trigonal prisms of sulfur; the Nb-S distances are 2.473 (1) A. From refinement of the common 0kl reflections the relative y and z positions of the two sublattices were found; along the common a direction the lattices of SnS and NbS2 are incommensurate.

J. L. de Boer

Papers

Diffractometer software (cad4f)

The crystal structure of the room- and low-temperature modifications of Würster's blue perchlorate, TMPD.ClO4. I. The room-temperature phase

The incommensurate misfit layer structure of (SnS)1.17NbS2, `SnNbS3'. I. A study by means of X-ray diffraction

The crystal and molecular structure of N,N,N',N'-tetramethyl-p-diaminobenzene–chloranil, TMPD–chloranil

The Crystal and Molecular Structure of N,N,N',N'-Tetramethyl-p-diaminobenzene Iodide (Wurster's Blue Iodide)