Question

A HF/6-31G(d) geometry optimisation and frequency calculation was performed for the Br2 molecule under standard state conditions (1 atm and 298 K). The thermal corrections were calculated based on the ideal gas rigid rotor harmonic oscillator approximation. Which of the following properties will change when the calculation is repeated at 0.5 atm and 298K? Select one: O a. The electronic energy b. The optimised geometry O c. The translational entropy d. The vibrational frequencies e. The rotational energy

Please explain your answer

Answer 1

THE ANSWER IS (b) THE OPTIMISED GEOMETRY

Because, here the temperature kept constant while pressure decreases. So it will affect the free energy of the molecule and gives an external strain to the molecular structure. The perturbation from this intial optimised geometry will induce the system to change so as to reduce this perturbation and to attain stability.