$$f = \frac{1}{T}$$

where:

* f is the frequency in Hertz (Hz)

* T is the period in seconds (s)

In this case, the period is 6.80 s, so the frequency is:

$$f = \frac{1}{6.80 \text{ s}} = 0.147 \text{ Hz}$$

Therefore, the frequency of vibration of the Citicorp Tower is 0.147 Hz.