How to delay a "not gate oscillator" to make it run at a desired frequency?
simulate this circuit &ndash; Schematic created using CircuitLab

This tends to work better with a Schmitt-trigger NOT gate (one with some hysteresis, or difference between "voltage it changes state on when going down" and "voltage it changes state on going up.") Circuit-lab's logic selection does not appear to have any Schmitt-trigger parts. Incidentally a Schmitt-trigger NAND with the inputs tied together also makes a Schmitt-trigger NOT.

It's a crude solution in any case and the R, C values are highly unlikely to give the precise oscillation you want. A 555-timer or 555 timer into a flip-flop is a somewhat less crude approach.

The frequency is one over the time for one full cycle. A full cycle is 1.5 seconds, so the frequency is 0.666666666666666666666... Hz (0.66666667 in most calculators due to rounding.)