I feel a little unclarity about this. Canonical ensemble is pictured as many systems in heat reservoir of infinite capacity having $N$ (number of particles), $V$ (volume) and $T$ (temperature) constant whereas microcanonical ensemble is the analogous system having $E$ (energy) instead of $T$ fixed. From my simple viewpoint of introductory pchem, it seems obvious to me that heat capacity is constant hence how is it possible for canonical ensemble to have different energies while having same temperature. Feeling temporarily frustrated I grab McQuarrie's pchem it reads on the topic:
"Each system has the same values of N, V, and T but is likely to be in a different quantum state, consistent with the values of N and V"
A light shines into my lower mood state. So we talk about very little changes where the concept of heat capacity must be taken with reserve? That is, it is possible that system sucks heat from this infinite heat reservoir which is used to promote a particle in the system to higher rotational state for instance, which causes the system to acquire a tiny bit of energy while temperature will not change at all? Temperature is understood as describing translational motion of particles, right? Is my idea of the difference between these to ensembles correct?