Quantum Chemical Investigations on the Structure, Bonding Orbitals, Frontier Molecular Orbitals and Reactivity Properties of Diphenylguanidine ? Vulcanizing Accelerator

Abstract

: The molecular structure and reactivity parameters of diphenylguanidine (DPG) were simulated with quantum chemical calculations by using the standard 6–31G**, 6–311++G** and cc– pVDZ basis sets. The bond distance of N1–C2 and N3–C2 are 1.39 Å and 1.42 Å, respectively. The N1–C2 bond posses much more double bond character than the N3–C2 with the longer C–N bond length, though both have partial double-bond character. The molecule exists in the more stable imino form. The C-N bond which connects the aromatic ring has a length of 1.28-1.30 Å, close to that of a typical C=N double bond (ca. 1.28 Å), and is shorter than the other two C-N bonds of the guanidine. The planarity of the (cid:1) tabiliza group is confirmed from the determined bond angles and torsion angles. The N1-C2-N3 bond angle (109.5°) is always smaller than the other two angles (N1–C2–N4 (123.2 o ) and N3–C2–N4 (127.2 o )). The MEP of the molecule spread in the range +1.224e × 10 –2 to –1.224e × 10 –2 . The range of total electron density of DPG is +5.545e × 10 –2 to –5.545e × 10 –2 . The bonding orbital for C2−N3 has 38.07% C2 character in a sp 2.24 hybrid and has 61.93% N3 character in a sp 1.94 hybrid orbital. This clearly reveals the partial double bond character of N1-C2 and C2-N3 bonds. The bond pair donor orbital, π CC → π * CC interaction between the phenyl ring carbon atoms are more (cid:1) tabilizat and the (cid:1) tabilization energy of these interactions lie in the range 16.97–22.78 kcal mol –1 . The large total dipole moment of DPG (3.208 D) shows that the polar nature of the molecule. The atoms C2, C13, C22 and C24 are most prone to nucleophilic attack while the electrophilic attack is more on C2, C19 and C12 atoms. The sites C2, C12, C19, C22 and C24 are more susceptible to free radical attack. Fukui dual reactivity descriptor ( ∆ f k ), the dual local softness ( ∆ s k ) and the multiphilicity descriptors ( ∆ω k ) indicate that the atoms C2, C9, C21 and C22 are more favorable for nucleophilic attack. The atoms N1, N3, C8, C19 and C23 are more favorable for electrophilic attack.