These notes are chromatically adjacent, i.e., they are the smallest musical interval, known as a half tone interval. Played alone, they produce an unpleasant, clashing sound.
Click to hear this sound as played on a piano: C & C#
Composers have, infrequently, used this type of note combination effectively, usually for brief moments transitioning to a more stable sound combination. There are examples of this in Beethoven's last piano sonata (no. 32).
Below is an example of this adjacent note combination over a longer time interval.
The overall oscillation of sound energy apparent from this graph is much too fast for the ear to detect, yet the net effect is considered dissonance.