Abstract
The <i>v</i>CH and 2δCH Fermi resonance doublet and unperturbed <i>v</i>CH and unperturbed 2δCH for 4-<i>x</i>-benzaldehydes increase in frequency as the mole % CHCl<sub>3</sub>/CCl<sub>4</sub> increases. One portion of the <i>v</i>CH and 2δCH Fermi doublet occurs in the region 2726-2746 cm<sup>−1</sup> and the other portion of the Fermi doublet occurs in the region 2805-2845 cm<sup>−1</sup>. The unperturbed <i>v</i>CH for 4-<i>x</i>-benzaldehydes is assigned in the region 2760-2810 cm<sup>−1</sup>. Submaxima are also noted for 4-<i>x</i>-benzaldehydes in the region 2726-2810 cm<sup>−1</sup> and these bands are assigned to combination tones. The <i>v</i><sub>asym.</sub>NO<sub>2</sub> mode occurs in the region 1533.60-1538.73 cm<sup>−1</sup> and the <i>v</i><sub>sym.</sub>NO<sub>2</sub> mode occurs in the region 1344.07-1345.63 cm<sup>−1</sup> for 4-nitrobenzaldehyde in CHCl<sub>3</sub>/CCl<sub>4</sub> solutions. The <i>v</i><sub>sym.</sub>NO<sub>2</sub> mode shifts to lower frequency by ∼4.8 cm<sup>−1</sup>, while the <i>v</i><sub>sym.</sub>NO<sub>2</sub> mode shifts to higher frequency by ∼1.6 cm<sup>−1</sup> in going from CCl<sub>4</sub> solution to CHCl<sub>3</sub> solution. The <i>v</i>CN mode for 4-cyanobenzaldehyde increases in frequency in a nonlinear manner as the mole % CHCl<sub>3</sub>/CCl<sub>4</sub> increases. The <i>v</i>OH mode for 4-hydroxybenzaldehyde decreases in frequency in essentially a linear manner as the mole % CHCl<sub>3</sub>/CCl<sub>4</sub> increases. The δCH mode for 4-<i>x</i>-benzaldehydes increases in frequency as the mole % CHCl<sub>3</sub>/CCl<sub>4</sub> increases; however, neither unperturbed <i>v</i>CH nor δCH correlates with Hammett σ<sub><i>p</i></sub> values.
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