Evidence of blue-shifting N-H center dot center dot center dot N hydrogen bonding despite elongation of the N-H bond

摘要

In most cases of X-H center dot center dot center dot Y hydrogen bonding (X is an electronegative atom and Y is any electron-rich center), the X-H bond gets elongated and the stretching frequency of X-H (v(X-H)) gets red-shifted. Relatively fewer cases of blue-shifting H-bonding are also known, where the X-H bond contracts leading to an increase in v(X-H). Here is presented the first experimental report of blue-shifting N-H center dot center dot center dot N H-bonding that accompanies an increase in N-H bond length as evident from quantumchemistry computations. Pyrrole and N-2 have been shown to form N-H center dot center dot center dot N hydrogen bond in an 'annealed' N-2 matrix, where both doped (pyrrole) and dopant (nitrogen) molecules had scope to align themselves and formstable H-bonded clusters. The infrared spectrum of the annealed matrix shows a blue shift of v(N-H) by 7 cm(-1). Computation of anharmonic frequencies at the MP2/6-31+ G(d) level of theory reflects the observed blue shift despite N-H bond lengthening after H-bond formation.